The Federation needing one year to replace the Wolf 359 fleet is the best starting point for an estimate.
To the contrary, I don't see any valid way to get relevant data from that. It's a peacetime event, we don't know if "the fleet" refers to Starfleet or the 23rd Fleet or whatnot, and we have no working definition of how "the fleet" is going to get shipshape - construction, reassigning or downsizing?
In contrast, DS9 is very explicit about all this, spelling out the answers.
We hear of exactly ten Fleets in wartime Starfleet, although we admittedly never get direct references to the 1st, 4th or 8th. There could be twelve fleets, perhaps, but if there are twenty-plus, then it's a major statistical anomaly that we only ever get references to 10 and lower.
We also hear how many ships there are in those Fleets. When the 2nd and 5th combine forces, there are 600 ships total. Sure, Sisko said this would be "elements" of said Fleets, but that was back when "elements" of the 9th were supposed to join the fray. 300 is a pretty good estimate for Fleet size, then, although possibly a lower-end one for the day. And the 7th having just 121 ships when going to major action in "A Time to Stand" may be taken to mean smaller Fleets at the early months of the war, or then an already badly hurt the 7th - we're still in the low-three-digits ballpark.
So, ten times 300 for 3,000 combatants. There may be plenty of support ships attached to a Fleet, but none were seen among the 600 in "Favor the Bold", so 3,000 would still be the number of actual combat vessels. How many would be available outside the Fleet organization, we don't know - for the "pseudo-WWI" style of fighting seen, keeping reserves would be strategically absurd, but OTOH Starfleet probably always has ships deployed so far from home on exploration duties that they miss any wars shorter than five years...
Another solid number we get is 1,500 Klingon ships against twenty times the number of Dominion ones in "When it Rains...". Makes sense, sort of - Klingons could well be level with Starfleet in numbers (their BoPs are smaller and probably cheaper and weaker, so the Klingons might be weaker overall), while the Dominion's massive numerical superiority would belie the fact that they operate plenty of tiny combat craft.
Timo Saloniemi