Without more accurate data about how long it took to rally the Wolf 359 fleet, my calculations yield a useless range of outputs.
If you assume it took a day and all the vessels came at warp 9, that gives a ship density of about 0.133 ships per cubic light year. Extrapolating outward over 100 sectors, that's over 100,000 ships.
But if it took 6 days at warp 9, then the density is 0.000616 ships per light year, yielding 500 ships in 100 sectors. (Starfleet reinforcements were 6 days away when Ent-D met up with the Borg just prior to the battle at Wolf 359, according to Memory Alpha, giving an upper.)
This range of answers is completely useless, as I am sure you can tell. Can anyone find closer set of (connonical) time limits?