I would like to know if there is a formula to calculate how much distance should I leave between enemies, or how many to put on the screen at the same time in a side-scroller game.
I feel like if I leave too few the game is going to get boring, but if I leave too many at some spot the game is going to get way too hard.
No. There is no such generic formula.
My experience in the professional game industry has been that tuning game elements (such as the density and placement of enemies) usually constitutes about 80 to 90 percent of the total work required to create a fun game. Most people would probably agree that it is also the most difficult part of the process to get right, and the most important to the final feel and success of the game.
Honestly, this is something that you shouldn't even want to delegate to the computer to do for you; too much rides on the decisions that are made in this area to ever trust them to a generic algorithm.
