I'd say going up to ~5 capitals per fleet would be fine, but remember that nearby fleets support as well, so too many capitals makes battles a little one dimensional considering how deployment works. If the major composition element is cruisers, it keeps all classes of ships more relevant in the late game, imo.
I will create a separate topic for this if it proves necessary, but radical idea incoming based upon the last few responses!
The player may feel the need to deploy as many capitals as they can if the enemy fleet generally doesn't have an overwhelming number of them too. The idea here is that the player assumes that would give them an advantage considering their oftentimes numerical disadvantage (arguable). Quality over quantity since quantity cannot be achieved.
To discourage capital spam by the player (completely arbitrary limiters are generally bad, but I don't
think this suggestion would fall into that category) would it be possible or easy to make it so that a skill is required to deploy one? So that a capital
must have an attached officer with the necessary skill to be usable in battle?
It would obviously be under leadership, and might make that more attractive and serve to partially balance out the power creep from the combat skill line.
Now, because that by itself would probably feel really bad or too limiting to a lot of people, the caveat would be that the player could use an undetermined number of story points to personally pilot a capital (or maaayybe to deploy one either without an officer or with an officer without the necessary skill) to circumvent that requirement with another resource cost to discourage doing it too often.
Idk, there are probably a lot of holes I haven't considered but the intent is to generate discussion.