Quick Google, at least for the US, but I imagine it's similar for the rest of the world. There are about 1 million licensed medical doctors and about 100,000 veterinarians.

It makes sense, people go to the doctor a lot more often than animals.

I'm not sure how this plays into it, but doctors tend to be much more specialized and many specializations wouldn't be useful in a zombie apocalypse, though their general knowledge would probably be valuable.

Rural doctors would probably do very well, though, a lot of them are going to have similar setups as vets.