We know that the Cardassian Eleventh Order--which was completely wiped out in the massacre at Septimus--had approximately 500,000 soldiers. The Eleventh may have been a ground unit; I never heard any mention of ships belonging to them. IF the Cardassians had twelve Orders (we never heard any number higher than twelve, anyway), and they were all of the same size--which we don't know, that would put them at full strength at 6,000,000 serving.
That would mean the Cardassians had LOST 1 million more already than their full forces are supposed to have. Now, hopefully for them, the Eleventh Order is the smallest of them all, and their full forces are supposed to be larger than that--but it also wouldn't be out of line with what we know about the Dominion using them for cannon fodder. That war had to have just chewed right through their population.