The round number indicates health of new zombies, and how many zombies you need to kill to progress to the next round.
Updates:
Update 1: This exists now and has been uploaded.
Update 2: Actual Zombies now. Woo.
Update 3: They can walk (animation added).
Update 4: Noticed some issues. Text placement fixed. Restarting no longer gives you 80 health.
Most importantly, saving is now a thing. Go and load and save. You know you want to.
Update 5: Zombie sight distance has been fixed.
Update 6: Something happened with the file or upload and last one was unplayable (update 5). Fixed confirmed.
Update 7: Health regen at round end.
Penetration.