really it depends on the map and the server.
most maps will have around 6(really small map) to 50 (huge map) vechiles.
when someone takes a plane for example and then they get shot down or crash, the plane will respawn after a minture or 2.
so you dont have to worry about running out of vechiles.
BUT! depending on the server you are in this can be a problem
people will sit and wait where the vechiles respawn. and people are not nice about it and line up and such they start team killling to get him the vechile and take off.
so sometimes you can have lots of fuin while in other servers you wont get 5 feet without being killed..