there's not really answer to your question. There is such a huge range in zoos, from tiny to large, struggling to secure, cowboy to professional. It also depends on where in the world the zoo is I should expect.
If you can find staff numbers for individual zoos, also, they tend to be for all staff not just the keepers.
I think it is safe to say, though, that few (if any?) zoos have enough keepers!