How do I destroy a sprite enemy after several hit?

I made an enemy sprite, and instead of destroying it with one shoot i want it to be destroyed after like 20 hits, how do i do that?

Use an instance variable to keep track of its life. Each time the player shoots it, subtract 1 from the instance variable. When the variable reaches 0 destroy the sprite.

im really new to constract, i apologize for my ignorance but,, i know how to make the instnce variable, but how do i keep track of it? in the event sheet i assume?