Help making a progression mob spawner?

Hi there, I have a new game that is in development and Im having a bit of trouble getting the code right for a part in this game.

I am trying to create a mob spawner that after the mob is killed a new mob spawns to replace it.

After 10 mobs with that amount of health has been killed I can then move on to the next lvl of mobs by pressing a button and upgrading the mob that spawns to have better loot and more health. (other properties)

example game would be egg simulator

I just need an example as I try to learn