Sets with the most zombies
Searching Zombies Minifigs on Bricklink I find 22 official Lego zombies, including the Monster Fighters, Minecraft, Scooby Doo, Ghostbusters, Pirates of the Caribbean, and several Mini figures from the collections.

http://www.bricklink.com/v2/search.page?q=zombie#T=M

Most of them are one offs in the set.

The most I found was three zombies in a set.  There were two sets:

Pirates of the Caribbean 4195-1 Queen Annes Revenge: Has three zombie pirates and a skeleton which I'm not counting as a zombie:



Monster Fighters 9465-1 The Zombies Set has Three Zombies, a zombie bride, a zombie groom, and a zombie driver: