Is there a max amount of vehicles that can be spawned at one time?

according to this result of the search for "vehicle limit" there is no known exact limit and atleast 30 vehicles at once work.

The only "limited" vehicles are commandvehicles (AT-TE, LAAT, everything that doubles as spawning point) since they count as mobile command post, and CPs are limited to 16 or so. But they can be easiliy turned into normal vehicles.
Afaik, the number of possible vehicles is only limited by what the engine can handle, not a fixed number.

Well, I thought so, but on my Hero Assault map, only 5 are spawned at once. I changed the SetMemoryPool for flyers from 5 to 10, but still no change.

The largest number of flyers possible is 32. More than that is impossible. If you try to use Lua to force spawn AI controlled flyers, they don't work properly. Most likely 32 is the limit for hovers and walkers.

If they're not spawning, it's some other unknown factor causing it.

They are spawning, but only when one ship is destroyed. Say I have 5 ships stationary, with a 6th one that should spawn, it only will if I destroy one of the 5.

Make sure there is enough space between the spawns, vehicles refuse to spawn if they don't have enough room for themselfes.
According to some chaotic experiment I conducted, 32 really seems to be the limit for walkers/hovers. Or at least a number close to that, it's hard to count stuff when everything around you is exploding. But every side should have had 34 vehs if there were no limits and they had way less than that.

Huh. Maybe it is a problem with the Hero Assault mode, since it uses all those animations for characters.