Counter of hostile mobs that the player has killed

I'm trying to create a mod that has a mechanic similar to the Solo Leveling comic, so I'm trying to create a global counter that keeps track of the number of hostile mobs I've killed, and then summon protective mobs depending on the amount I've killed before.

For example, if I kill 10 zombies, I would like to be able to use an item or a key to summon 10 'Shadow Zombies', and if I kill 5 skeletons, I want to summon 5 'Shadow Skeletons'.

I have the issue of protective mobs resolved, but I'm struggling with the events since I'm not familiar with them.