I usually have 2 hangers per base for the majority of the game, up to 8-10 in total. The main base will very often run 3 in the first 2 years simply because all the best gals are already there so having your underwater craft there just makes sense, until the number of available experienced/trained gals rises and you can move the underwater crew someplace else.
3 hangers are transports, one for space, underwater, and large capacity craft for general work. Depending on tech level and codex choice you can, reduce this to lower numbers by using multi environment transports, but you usually dont want fewer then 2 cause the micromanagement of everything being on one craft gets aggravating, not to mention risking more gear in your item pile.
The other 5-7 are intercept craft. 1-2 fighters with weak light cannons and thrusters for small ufos, 1 heavy tank craft, 1-2 anti shield interceptors, and 2 anti hull interceptors.