I was a bit confused at first, but I love the concept for this! I really like the idea of choosing your own challenge rather than being given pre-determined waves. I would love to see an incentive for hitting certain goals, like unlocking a new upgrade for spawning 10 zombies or something. It did feel a bit unintuitive having to type the number I wanted, maybe having + and - buttons would be more clear. It would have also been nice to have an indicator of where the shooters' bullets were going, just to make reacting to them a bit easier. I did find a bug where dying and restarting disables the other enemy types in the spawner but if you don't reset them manually they will still spawn the same way they did when you died. Overall, very fun concept and great job!