It depends on your system.
For instance, if you are trying to keep SPS and other corals that require low nutrient tanks, you will want to stock lightly. You might only want to add one more small fish to that tank. If you are planning to go fish only or LPS/Softie tank, you could add two or three more fish. It also depends on how often you change the water in your tank, whether you are running GAC and GFO and what kind of skimmer you have.
I strongly recommend the Scott Michael fish book. He goes through many of the main groups of fish and gives the minimum tank sizes. As a rule, the type of fish that could be kept in a 34 gallon aquarium include: most gobies, many smaller blennies, firefish, fairy wrasses, and basslets.
When I had a 29 gallon tank with mostly LPS and softies, I stocked it with a royal gramma, a tail spot blenny, a purple firefish, a clown fairy wrasse and one blue-green chromis.