A very good question, they obviously upped the tempo in the Dominion War compared to BoBW where 40 ships was considered alot!
Plus Way of the Warrior said the Klingons had only about 100+ ships that headed to Cardassia which was about a third of their total forces c. 2372
My SRM doesn't contain any information about how many ships the Federation can produce per year. I discuss the size of the fleet, and the impact of the Federation economy on the conduct of the Dominion War, in the DOMINION WAR SOURCEBOOK, but that's as close as I come to the issue.
How many ships does Starfleet has anyway? 300 ships for the Klingon Empire seems awfully small. It would mean their fleet is spread thin throughout the empire. Is it the same with the Federation?
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"As long as you can laugh, you are not
defeated."
Remember that was 2372 before the war started. In 'When it Rains' (late 2375) the Klingons have 1500 vessels available to deploy. SInce this is towards the end of the war and the KLingons are heavily envolved, i would say this represents over half their fleet, perhaps more since rommies are tied up in the war as well.
The feds of course had c.627 ships between 2 fleets for 'Favor the Bold' so perhaps their fleets were 100-400 ships each and there was 10+ fleets. So c. 3000? more/less???