My preference on this matter is that the Borg don't reproduce, period. They assimilate, and then they assimilate more - but not with the intent of keeping up a population. The assimilation is performed (almost) solely in order to bring more of the miserable unconnected individuals under the loving care of the Collective.
In banal terms, this happens by punching needles through their necks and adding bits to their bodies. But the body is just a temporary stage there, and not even a particularly valuable commodity for the Collective - even though they don't waste the resources a body might offer. Sooner or later, the Drone is expended. But the assimilated individual continues, as an echo in the Collective, eternally alive in there unless for some reason purged. Any of these physically deceased individuals can be accessed by any of the physically remaining individuals, even, much like Seven accesses a variety of personae in "Infinite Regress".
At times, the Borg might suffer an influx of Drones, at times a drain. There's enough buffer capacity there to withstand a Species 8472 war or two, though, and never a pressing need to create or acquire more Drones just for the sake of Drone body acquisition. Babies and pregnant females or female-equivalents are quite often assimilated, for the richness of experience that their addition to the Collective provides, or then as collateral acquisitions; the Borg have no particular reason to abandon a potentially assimilable individual, no desire to fail to bring it to the paradisial realm of the Collective, and while they can put certain less appealing individuals "on hold" (say, the Kazon), they probably won't turn down a collateral baby because he or she cannot be similarly put "on hold" once the parents are assimilated.
Timo Saloniemi
That's a very compelling argument for no Borg reproduction. Ironically your first paragraph pretty well sums up my reasons for preferring them to assimilate only for self-improvement and reproduce for drones, but, if they do reproduce, I suppose a more logical extension of that thinking is that they wouldn't want to leave control of their reproduction to chance, so selective assimilation could provide a better means of troop replenishment.
You also make excellent points about the body itself not being the target, but I suppose they do need the bodies to do work on the ships? I guess the question then becomes how many drones do they need to do actual physical work that can't be handled by nanoprobes via regeneration? I recall from 'Dark Frontier' that the Borg have medical drones whose purpose seems to be repair of other drones, and we know they have tactical drones, so in that light perhaps the duties of drones consist primarily of repair of other drones and assimilation and actual ship maintenance is not something that requires a lot of their attention? So reproduction might not be a necessary factor for the Borg, since they can replenish their needed supply of drones to your proposed threshold by their already existing assimilation goals?
Ok, I think you won me over.
We don't know that one ship survived. We know that Admiral Hanson wished that 40 ships could attend, but that he was hoping for more; we know that 39 were lost. Also, we know that there were surviving personnel from the battle who were able to escape from the site of carnage before the Enterprise arrived.
From that we can deduce that at least one warp-capable vessel, but perhaps dozens, indeed left the battle, carrying survivors like Ben Sisko. It would be arbitrary to claim that only one ship escaped; it would be strange to say that more than a fourth of the total strength of the fleet fled. The truth is probably somewhere in between.
OTOH, many of the ships destroyed in the battle may have survived. Sure, all the crew were killed or assimilated, and the hull was full of holes, but it was still a hull; it could have been repaired. Similarly, the US Navy tried to create the impression that no battleships were really lost at Pearl Harbor: all the sunken and destroyed ones save Arizona were raised and repaired or, if necessary, almost completely rebuilt so that it could be claimed that they had been salvaged.
One of the background wrecks in "BoBW" was labeled the Ahwahnee; a ship of this name and (slightly misspelled) registry would later see service in the episode "Redemption", possibly because she had been salvaged and repaired. Many of the almost intact wrecks might have been similarly restored.
Timo Saloniemi
These are also excellent points. Starfleet does seem to play fast and loose with their numbers don't they, not just the Prime Directive? Your assessment of them is spot on, and the comparison to Pearl Harbor is quite appropriate. I've never felt that there were 40 ships at Wolf 359 and only one happened to survive. I've always felt there were something like 47 )there, given Starfleet's fondness for that number.
According to
Ex Astris Scientia there were at most 15 wrecked ships seen on screen in the episode representing those destroyed, plus the four ships seen in 'Emissary,' two of which we actually see destroyed (Melbourne and Saratoga - I don't believe we actually see the
Yamaguchi and
Bellerophon destroyed on screen). In the case of the Excelsior model seen in 'Emissary' technically replaces the Nebula prototype seen in 'Best of Both Worlds' so the regular Nebula shown in 'Emissary' (the
Bellerophon according to
Memory Alpha) could be argued to be the Melbourne wreck from 'BOBW'. The Memory Alpha article also suggests that the
Yamaguchi (the
Ambassador class ship from 'Emissary') was destroyed but I don't know we have any confirmation of this? In any case, that leaves us a lot of leeway - a maximum of 18 wrecked ships seen, 39 total based on dialog. If I recall correctly, the 39 number comes from the
Enterprise's assessment upon arriving at the battle site? I wonder if the other 21 detected were whole wrecks like the others, or just a chunk of primary hull with a transponder in it there, a warp nacelle with a registry number there.
Let's arbitrarily say there were 47 Starfleet ships present, 39 lost. That leaves 8 ships we don't know the fate of. Maybe five were captured by the Borg and deemed worthy of assimilation and sent back to Borg space with drones, and maybe another three escaped with warp drive intact and were able to pick up lifeboats and flee, no doubt in different directions to discourage the Borg from following them. We know that the 18 we saw were relatively intact, so those could have been repaired or rebuilt into service, as probably happened with the
Ahwahnee. From dialog in TOS 'The Doomsday Machine' it seemed like the
Constellation (which was indeed described as 'wrecked') could have been fully repaired if taken to the proper facilities. I mean, at Wolf 359 the
Melbourne had a big chunk of its saucer vaporized, but why couldn't they just replace it with another saucer? I'm sure it wouldn't be that simple, and it would be a bit morbid, but really, why not? In fact, isn't it possible that rebuilding those ships was what Cmmdr. Shelby's duties were going to oversee when they referenced 'rebuilding the fleet' at the end of 'BOBW II?'
And those are just Starfleet ships - if the Klingons (or even Romulans if they ended up being asked) arrived, that complicates the scenario even more. I personally believe there may have been some Klingons present in some capacity but I doubt the Romulans would have sent any ships - the Romulan seen in 'Unity' notwithstanding - he could have been assimilated by the Borg in their destruction of the Romulan colonies along the Neutral Zone). If the Romulans did sent a warbird or two it might have been more likely to sit by under cloak and watch.

But either way, I'd say no Romulans in the fight.
Regarding the Klingons, there are two possibilities if they came: they arrived soon enough to participate in the battle, and were possibly among the wrecks detected, or they arrived in time to help pick up survivors? If they arrived soon enough to participate and were among the destroyed, I think that thows the 39 number into question of being only Starfleet ships, possibly there was even a Vor'Cha or other Klingon ships captured and sent back to the DQ? I mean, General Korok from 'Unimatrix Zero' had to come from somewhere, right? He could have come from an off-screen battle but who knows.
And regarding the
Endeavour mentioned in VGR's 'Scorpion' being the '40th ship' (feels like being the second gunman on the grassy knoll or something) I just don't think it was the
only surviving ship of Wolf 359, if indeed it was there, which we don't know. We just know that Amasov encountered the Borg and Janeway was studying him when studying the Borg. If the
Endeavour somehow escaped a Borg cube by itself, that would almost put them on a level with Our Heroes on
Enterprise and
Voyager since only they have appeared to do the same. We know based on 'I, Borg' that Borg scoutships were in Federation space from time to time, so how do we know Captain Amasov didn't encounter one of those?
So yeah, definitely more than one surviving ship, and no reason to think those we saw on screen as wrecks weren't rebuilt.