From personal experience of doing a bunch of interviews and surveys over the years, after about 6, you'll start hearing patterns. If you are new to the subject you are researching, doing some reading and talking to about six people will get you to a point where you'll be able to evolve the questions you are capable of asking, or formulate hypotheses for testing.
"How To Measure Anything" has a great chapter on how talking to only a few people can reduce uncertainty with a pretty amazing accuracy, but I don't have a copy handy.
"How To Measure Anything" has a great chapter on how talking to only a few people can reduce uncertainty with a pretty amazing accuracy, but I don't have a copy handy.