What determines the number of zombies attacking the town?
How is the number of zombies attacking the town at night determined?
I don't think a formula is known, but here is a website that has collected stats from the French version of the game and gives a min/max/average attack for each day.
8/10/11 Update: The new version of the game has significantly changed the way that attacks are generated and resolved, so the web site above is not going to be helpful anymore. It would be great if a new answer could be provided with new details.