What DAGGZODER said is all true, but if you finished the game like I did, and liberated all the outposts, it will be kinda hard to find that many (50) enemies.
However there is one place where they respawn. Its the airport on the seconde island. There is always 10-15 guys around plus outpost is very close so its easy to just fast travel there, buy ammo, go back and kill again. hope it helps.