I don't believe quantity of defects found, should ever be a goal or concern in the context of your question. The most important thing should be quality as opposed to quantity. When I say quality in this context, I refer to the robustness of the your defect detection methods. If you are reviewing documents and designs, inspecting code and, testing without finding defects - I would suggest that you need to review your methods.
Metrics play a big role here.
One should be watching for good or bad trends of defect discovery. If the tester is testing a particularly buggy piece of software, with bugs that are easy to reproduce, easy to write into defect reports, then perhaps quite a few. If the tester is working on a stable piece of code, and doing mostly regression testing, then maybe none. But you never know. I once worked on a build that crashed my machine and deleted system files.