There's around 8 enemies per stage with the exception of the boss fights.
Stage usually starts with 4 spawning in. Then 2 and finally two. The game marks where they are going to spawn so you can just stand near one for a quick kill.
I do a full run-through from about 12:30 in the video. The video is precisely how long it took for me playing for the first time to the 1000g completion. I left the first half in to show a reflection of how easy it was to complete.