No limits. Once triggered, an outpost will spawn a maximum of 10 outposts at a time within 10 clicks of the tunnel (Not within 10 clicks of the player base as with camps). There is a variable spawn rate so in theory, one could kill all the outposts before any respawn though typically, this does not happen unless the spawn rate is too slow. I don't understand the 'low level' question, if a player base is not high enough to spawn outposts then nothing it does will affect outposts. More player bases in range that qualify a tunnel to trigger and the varying level differences all affect outpost spawn rate in my experience. Generally speaking, you can sit there and farm outposts until you run out of CP/SP or RT.