I'm not statistically worried about us never hearing about those three "missing" fleets. It's not as if we heard of the other fleets very often, either. All the numbers from 1 to 10 could very well have been in active use in the statistical sense.
However, since there were absolutely no mentions of fleets higher than 10 in number, but there were relatively numerous mentions of fleets lower than 10 in number, it's statistically quite dubious to postulate the existence of twenty or fifty fleets. That is, unless we also
postulate some reason for why Starfleet would predominantly use the first ten fleets and leave the others to do indistinct stuff somewhere off camera.
What we've seen often in Star Trek is they tend to prefer using Task Force as a way to designate a group of ships more often than fleet.
And a Task Force seems to consist of mere dozens of ships - which TNG "Redemption" already establishes as far too low a number to perform offensive operations. So both designations probably have their place in the Starfleet terminological hierarchy, which in this sense appears to closely resemble the good old US Navy one.