Well, prior to the Dominion War, we know of at least four, the Defiant itself, the Valiant and the three seen in A Time to Stand. Considering we only know of six Defiant class ships total seen on screen (the other two being the Sao Paulo and one seen in the fleet at the end of Endgame), that means roughly 65% of the class that's known was definitely built before the Dominion War started.
But in those giant fleet battles that we saw, how many "Defiant Class" vessels did we see in a fleet?
Usually, only 1, maybe 2 or 3 on special occaision.
Where were the rest of them?