Hi Timmyvos,
It is our express goal that the game will be perfectly playable on normal PCs. Last time I checked, it ran fine on a 3+ year-old Dell xps m1530 laptop.
The size of the fleets will probably be determined more by what makes a good and fun game than the system requirements. Dozens of ships and hundreds of planes works very well in the current pre-alpha (when the shooting starts, add hundreds of missiles). It's certainly enough work for the player to keep track of everything.
It goes without saying that in a mod or skirmish, you will be able to add ridiculous amounts of units, and sooner or later system requirements will be a limiting factor. Also, of course, network performance when you play over the internet.