I think 5 customers is ok as long as you feel they are saying exactly the same thing. Almost no difference. If you have 5 conversations and they are similar but not repetitive enough I would talk to more people. You almost want to know that those 5 people could sit in a room and node at each other if they were talking about the problem they have. If that is not the case, maybe you need to talk to more. The number is arbitrary, is more about how many people you need to talk to in order to hear the same thing over and over again 
Does that make sense?
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