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How many ammo does it take to stop zombies?
The designers answered every part of this between 2019 and 2020.
One ammo stops one horde on one side of one street. Stopping both sides of a street takes two, and a double (two-arrow) card needs up to six to stop everything. There is no limit on how much ammo you may spend, and you can split a total across several Zombie cards. If you stop a horde with ammo, you do not draw its arrow to the next hole. A City Plan that asks for five ammo means five in total.