The number of interviews of participants you might need to engage with in a round of qualitative interviews to achieve data saturation can be difficult to estimate. But, there are a few variables that usually factor in.
First, you need to consider the domain of the research. When you are conducting research on an established product or a domain in which you have well established knowledge, you'll likely reach the point of saturation more quickly. In such cases, you can get by with fewer participants.
Second, think about the breadth and diversity of the participants. If you're focused on a narrow, homogeneous group, you won't need to engage with as many participants as if you are trying to learn about a group that has a number of distinct needs.
Finally, a more experienced researcher can do more with less. With qualitative research, the richness of the data is more often a factor of the researcher's skill than the sample size. They've honed their interviewing skills and are adept at asking the right questions and determining on the fly where they need to dig in further.
- UX researchers should care about strength of evidence
- Qualitative researchers care more about data saturation than sample size
Seaman, Mitchel. “The Right Number of User Interviews.” Medium (blog), October 5, 2015. https://medium.com/@mitchelseaman/the-right-number-of-user-interviews-de11c7815d9.
Yocco, Victor. “Filling Up Your Tank, Or How To Justify User Research Sample Size And Data.” Smashing Magazine (blog). https://www.smashingmagazine.com/2017/03/user-research-sample-size-data/.