When I stress-tested the engine, I noted that 2 active enemies on screen at a time is ideal... 3 seems to be the ceiling. 4 or more and you start getting "brainless" AI and a severe dip in framerates while they struggle to do anything other than walk towards your general position.
It's going to be tricky, but you will have to get used to using triggers to spawn your enemies selectively around a player. A simple way is to build a dead-end alleyway somewhere, and at the only entrance/exit, set a one-time event to spawn two bad guys out of sight in the alley. Once they're dead, they're dead. Moving all around the map, theoretically, you can have hundreds of enemies in 1 - 3 enemy bursts with no hit to overall performance.
If used imaginatively, you can come up with several pockets or buildings or valleys, etc where you have enemy spawn points that release a total of (example) 20 enemies each from an area unseen to the player that flood in one after another after an enemy has been killed by player. (Serious Sam is a series famous for this type of enemy dispensary).
EDIT: In the case of your video here - To start, you can have a couple of enemies pacing the street... that's inexpensive. They get shot, they despawn, then your resources are free for 2-3 more enemies once the player crosses the spawn trigger zone. Consider placing those enemy spawn triggers at the locations you are intuitively hopping between for cover from bullets... the new spawned enemies can appear from buildings, alleyways, behind vehicles, on rooftops - use your imagination! Create those scenes where the player can "feel" a fight coming... then deliver it.
http://www.imdb.com/name/nm2831087/
