Hey, thank you for playing! They drop pretty rarely at the start, but after around wave 7 the drop rate speeds up a lot. There are 16 waves in total.

I know this probably should’ve been explained somewhere in the game, but I was running out of time before the deadline.

Thank you so much! It honestly means a lot to hear that. This was actually my first game, and it took a lot of work, so reading comments like yours makes me even happier. I'm really glad you enjoyed it, and it's awesome to hear you had so much fun that you're already going back to try different upgrade paths. Thanks again for playing and for taking the time to leave such kind words!