Thank you for playing it! The number refers to a sort of "wave" number, though the wave delay is something like 8 seconds from the last zombie spawn. In practice, it ends up being a more continuous experience lol.
That snowball reload mechanic is exactly the kind of thing I wanted to implement but didn't have time for. Similarly, I wanted to make the snowman rebuild the snow fort (i.e.. heal) after idling for some amount of time. But, that's just how jams go, I guess!