There's around 8 enemies per stage with the exception of the boss fights.
I recommend using the Flamethrower Hat or the Exploding Bomb Hat to deal more damage. The Triple Shot Hat is also useful for the weaker enemies.
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.