Well, there is always a risk of leaks, with more people or less.
Of course, but risk is something that has to be managed. If all your beta testers are employees, risk still exists, but the risk level is less than if you are picking only strangers. By the same token, the more testers you have the higher the risk, since each person adds more risk. The fewer people who are in the test, the fewer people who are able to leak, and furthermore, if there is a leak, fewer people means it is easier to pinpoint the leak.