How do I make waves of enemies

How do I make waves of enemies like in wave one spawn x amount of enemies and when those enemies are destroyed go to wave 2 and spawn more enemies than wave one.

Maybe something like a variable that counts the amount of enemies.

e.g in wave 1 there should be spawning 20 enemies. so when the variable that counts the enemies reaches 20 you stop spawning. And when it reaches 0 you go to the next wave. Then in wave 2 you stop spawning at 30 enemies and so on

watch this guy make a tower defense game