Increase number of enemies spawned per level

Hello everyone, I'm a novice programmer and I'm making a game as a project for a class that I have. The script below is what I'm using to spawn enemies, but what I want to do is spawn a specific number of enemies per level. For example initially I would like to spawn say 10 enemies, then pause the spawning for say 10 seconds and then spawn 20 enemies, wait ten seconds, spawn 30 enemies and so on. I'm thinking that I'll probably need to set up an enemy counter(which I don't know how to do either) and then a for loop to increase the number of enemies for each level. I might be totally wrong in my assumption, but if anybody could provide any help on how I should modify my script I would be very grateful. Btw if it's not obvious I'm writing the script in C#.