Christopher said:
^^Well, that's the largest Starfleet vessel class that we've seen. Not all Federation ships are Starfleet ships. There could easily be civilian liners or colony ships that could hold larger numbers.
Also, 1000 people is only the default complement of a Galaxy-class ship on a typical exploration mission, in the "luxury liner" mode we saw on the show with abundant extra space and power. If a Galaxy-class ship were configured in more of a Klingon way, a bare-bones military situation, it could hold far more people. According to the TMP Tech Manual, there are enough unoccupied quarters available to support a crew of up to 6,500 people in the short term. And in an emergency evacuation situation, with cargo and shuttle bays converted into living space, a Galaxy-class ship could hold up to 15,000 people.
I expect that all those Galaxy-class ships we saw in the Dominion War fleets were used as troop carriers, ferrying infantry in massive numbers. I can't see any other reason to keep those bloated saucer sections attached rather than going with just the battle hull alone. And come to think of it, the ship in "Yesterday's Enterprise" seemed far more crowded than it did in the main timeline, so I wouldn't be surprised if it had a complement of 5-6000.