How do make bosses appear

Hi guys I wanted to know how do I make bosses appear my game is going to be endless but I want a boss to appear after X amount of normal enemies and also increase that X amount as player progress

You can make a global variable in your event sheet called Bosses (or any desired name) and keeping the Type as Number with value set at 0.

After an enemy is defeated add 1 to the Bosses Global variable.

If Global variable = 3 (or desired number) spawn Boss 1.

When variable equals 20 (or desired number) spawn Boss 2 and so forth.