The writers throw around those sorts of numbers, but they don't seem to jive with everything else from the franchise. At Wolf 359 the Federation "only" lost 39 ships, but it was seen as a major disaster that greatly weakened them and required them to accelerate their ship-building programs - and even then, it took them several years to recover to previous strength levels. Wolf 359 was in 2367, and the Dominion War started in 2373. It just doesn't seem at all plausible to me that it took the Federation several years to replace 39 ships, but then just a few years later they were suddenly able to jump up to producing tens of thousands of ships/year. Sure, they would certainly have started crash building programs once it became clear that they were going to need a lot of ships for the war, but come on...that would require them going from being able to build around 15-20 ships/year to many thousands of ships per year. Even within most the DS9 events of the Dominion War, tens of thousands of ships doesn't seem to make much sense. For example, when the Cardassians and Romulans launched their sneak attack on the Founders homeworld, they only had 20 ships in their fleet. If the war really involved many tens of thousands of ships, that would be an insignificant drop in the bucket that could never possibly have accomplished anything. They were very surprised to find 150 Dominion ships waiting there for them. If the Dominion really had tens of thousands of ships, they would surely have expected far more than that to be guarding the Founder home planet...I don't see any way you can reasonably extrapolate them being surprised at 150 ships defending the Founder planet to a fleet of 50k-120k ships.