0.5.3a, the last release before skills, player had a 100 FP limit. That is about twenty frigates or two or three capitals or somewhere in between. Can be exceeded but either at the cost of more supplies or risk of accidents. Biggest NPC fleet was Hegemony System Defense Fleet which had three Onslaughts and a variety of ships.
0.5.4a (I think) introduced skills. Unskilled, you had 25 FP base, enough for five(?) frigates or one Paragon. Leadership attribute and Fleet Logistics added FP, up to either 100 or 120 FP (do not remember).
0.6a added CR and Logistics. Unskilled, you had 20 FP base, enough for four or five frigates or one Conquest. Other capitals required Leadership just to get enough FP to pilot it only. Also, personnel ate into Logistics. The more crew you have, the less ships you can bring. Player could exceed Logistics somewhat at the cost of max CR. With max skills, and breaking the limits a bit, player could bring about forty frigates and kill everything or three capitals.
0.7a did away with Logistics and Leadership based limits, and added the fleet cap, which began at 25 ships... for the player only. (NPC fleets violated this limit frequently.) Now, frigate mobs could not be as big as they used to be, but you can bring a bunch of capitals if you wanted. (Before, you could only bring about three capitals and little else.)
Keep in mind that until 0.8a, fighters were treated as ships and counted toward FP/Logistics/fleet cap. Most fighters were the equivalent of frigates, but few like Warthogs, maybe Tridents too, were closer to destroyers. Fighter spam could be one or few carriers and the rest of your fleet were fighters.
Sometime later, either 0.8a or a later 0.7.xa release raised the cap from 25 to 30 ships.
0.8a changed fighters from ships to missiles, and they no longer count toward the ship limit.
0.9.1a, NPC fleets usually honor the fleet cap. Instead of a hundred frigate horde, they send eight to ten capital fleets at the player.
There's only so much a single fleet can do.
Especially when player cannot deploy everything due to battlemap size. 300 FP is not enough to deploy all big ships player may want.