This will come naturally as well. Everything you can kill counts as an enemy, so kill away! You'll definitely reach this before the game is over since all areas respawn after a certain amount of time and you have to cross each area several times to get where you need to go and do what you need to do. If you want to know how many enemies you have killed so far, just go to the leaderboards in either the main menu or pause menu. When it's loaded up, press and you'll see your name pop up as well as the number of enemies you've killed thus far.