Depends. If the combat is realistic (ex. COD) , I'd say about 10. If it's unrealistic (ex. Halo), just make sure they don't crowd the screen. Also depends a little bit on the enemy. If they're some kind of dumbass enemies that get killed in one shot and spend their short lives running in circles and missing every shot, you can definitely have more, but if they're some kind of man-refrigerator hybrid with an IQ of 210 and some kind of inhuman accuracy, then you obviously can't have very many.