they spawn indefinitely, the faster you can kill enemy's the faster you level up. the later levels get harder as you progress in the game. The gold gain is very slow to,makes it kinda difficult to progress as well. If you get Thor's hammer it makes the game like way easier as it does like 50 damage but the spawn rates for that weapon are low. I got it once testing the game for Like 10 hours. You can equipt different things to buff your character as well