But 1200+ ships on one side of one battle ? That's like two orders of magnitude bigger than the next biggest battle prior to that.
The Federation had never been at war for an extended period during a TV series before Deep Space 9
. There was never a need to show that many ships. It also didn't hurt that computer technology had progressed enough by 1997 that using digital technology to insert ships into battle scenes was possible. Had the Dominion War happened during The Next Generation
, it's doubtful a large-scale battle could have been filmed.