In the early stages of a game, one zoo keeper can take care of 3 or 4 exhibits, if they are close together. As the game progresses, and the animals have babies, there are more animals, therefore more feeding, therefore more poo, & each exhibit will require more attention. Usually one keeper can still handle two 100-square exhibits. When you get into the larger, multi-species exhibits, you may need to have more than one keeper per exhibit. They still need to be able to exit the exhibit, or they will just stand there going "duh".
To give your keepers a little help, try Loewenfrau's Clean Up Hack. You can manually delete poo, and get $64 instead of the $50 from the compost heap. (And you can buy a nice Parliament CD for that extree $14.) But be careful. I find that if you delete a poo that a keeper was about to clean up, the keeper sometimes goes into a trance and does nothing until you pick the keeper up again.