Five zombies. Four bullets. Two zombies.
This was on www.sixwordstories.net . I interpreted it as the guy shoots 3 zombies and then shoots himself, but someone else on the site thought the guy used up all the bullets and then was bitten and became a zombie. The latter makes more sense if the guy didn't know how many rounds he had to begin with, but I like my darker and (arguably, to me at least) funnier interpretation.