heh, realism = no, it'd be pretty boring if a 12-house town only had 50 zombies in it :)
If it makes you feel any better, consider the zombies could represent the total number of bodies in the town, not just the living ones. I'm a bit skeptical about thousands of zombies, as the largest cities in the game don't have a population of 2,000. If there were thousands of zombies in a town that small, it was a bug. I'd expect 600 or so at the upper end.