A couple of details:
We hear of seven fleets only: 2nd, 3rd, 5th, 6th, 7th, 9th and 10th. There are two main possibilities here: either all fleets from 1st to 10th exist, or then only the seven mentioned here do.
Of course, either option can accommodate a couple of additional fleets. Statistically it's possible that we missed mention of the 12th Fleet. But it's less likely that we'd have missed mention of all the fifteen fleets from 11th to 25th.
The average fleet size is probably larger than 200, because the 2nd and 5th Fleets were between themselves able to contribute some 600 ships to retaking DS9, without apparently being present in their entirety (the 2nd had been recently weakened, and in any case Sisko spoke of taking "elements" of the two fleets into action). The 9th was supposed to contribute another 300, apparently, but didn't make it in time.
Possibly all the fleets started out understrength, waiting for the arrival of dispersed assets (vessels on exploration missions) - thus, the 7th would feature barely over a hundred even when going in, and would come out gutted to a tenth of that. Ultimate fighting strength might have been at about half a thousand ships per fleet.
We might also speculate that the fleets were dissimilar in makeup. The 9th that we most often observed might have had more than its share of antiquated vessels (considering that it was deployed in an area of little activity after the wormhole lost its strategic significance, yet close to the enemy so the most modern warp engines wouldn't be essential). All the cool modern designs like the Galaxy
-kitbash types from "BoBW" could have been in other fleets that operated elsewhere across greater distances.