Three things that matter more than the number
The right people
People who just went through what you want to know about, like a first purchase or stopping recently. Not anyone who's free.
The same questions
Ask everyone about the same thing and follow up the same way. Then you can compare the answers.
Listen for what repeats
Things that come up by themselves, from several people, without you putting words in their mouth, are what to pay attention to.
Judge by what you hear, not by a number
- Later people tell stories you've already heard
Nothing new from several people in a row: you can stop.
- Everyone tells a different story, nothing repeats
That's an answer too. The question is too broad, or you talked to the wrong group. Fix the question or change the group, and talk again.
- You have different kinds of customers
For example, regulars and people who stopped. Talk to several of each, not one of each.
One person is one story
One customer who complains loudly, or praises a lot: listen to the end and follow up, but don't change the whole shop for one voice. If you change for every voice, the shop changes every week and nobody is happy.
Talk about numbers the way you really talked
If the question has clear answers, like why they come, you can count how many of the people you talked to answered which way. But the people you talked to aren't all your customers.
90% of customers come because it's near their office
9 of the 10 people we talked to said they come because it's near their office
Ten people can't tell you a percentage of all your customers.
Half our customers want a breakfast menu
4 of 10 want it, 3 don't, 3 didn't answer clearly. We can't say yet.
A split like this means talk to more people, or accept you don't know yet.
Questions
Is five people enough?
It might be, if all five are the right people and what you hear starts to repeat. If it doesn't repeat yet, keep talking.
Can I ask friends or family?
If they're not customers, their answers tell you little. And people close to you are even more polite: they praise more than they tell you straight.
The answers all differ. Was it a waste?
No. It means the question was too broad, or you talked to different kinds of people mixed together. Good to know before you act.