I found these papers very informative in this respect:

http://www.qualitative-research.net/index.php/fqs/article/view/1428/3027

and

http://johnlatham.me/many-participants-enough/

Hope this guides you in the right direction.

You could use the "Search" function here to find several other questions on this topic. In general, the recommended approach is to reach 'saturation" rather than to apply any formula or general rule of thumb.

Agree with David, above; 'saturation'---when data being obtained from respondents is repeating--is an indication to move one's project on, either by seeking new data from respondents, or concentrating on a new phase of the project.

What you might do actually depends on your research design.

The following papers should be helpful to your topic: 

• Guest, G., Bunce, A. and Johnson, L. (2006). How Many Interviews Are Enough?: An Experiment with Data Saturation and Variability. Field Methods, 18, 1, pp. 59-82.
• Hennink, M. M., Kaiser, B. N. and Marconi, V. C. (2016). Code Saturation Versus Meaning Saturation: How Many Interviews Are Enough? Qualitative Health Research, 27, 4, pp. 591-608. 

Furthermore, you may also refer to some more references in the following link: https://www.researchgate.net/post/How_many_interviews_are_enough_in_qualitative_research_case_study_studies/1 

Dear Yael,

Definitely, you would need to collect sufficient depth in your evidence until you reach to empirical saturation. But, remember this is may not always be the case, because it depends on your research questions in the first place, then your research purpose. Make sure what exactly you expect from your evidence; do you want to support your conclusion or …

One important factor is to know what really constitutes excellence rather than adequate in the field of our research.  If you really mean of collecting large significance data, well, conduct as many interviews as needed to achieve your goal.

When you will get repeated answers from the respondents then it's over. It means you are done with your query.

It depends on your methodology, different ways of eliciting lived experiences require different numbers.

I've tended to use 12 as a benchmark but that's my personal number.

Dear Yael,

You continue interviewing as long as you keep getting new information from the interviewees. You stop when no new information is forthcoming, i.e. you have reached a stage of 'data saturation.

Thank you for your responses. Both the notion of saturation and actual numbers (12, great!) are helpful. I know there is a difference in methodologies between areas of research but as I need to plan for a stay in another country, I wanted to know how many interviews to plan ahead - 10, 20? I will do about 10 in my country and then travel to do more abroad. 

Yeah, I agreed with Bahram Shahedi and Noman Muhammad. If you still get a new information from the participants, It's better for you to continue the interviews and make sure that you get the information that you need it. Determining the number of sample in qualitative study will only make you lose the opportunity of getting pearls.

Thank you. I heard to use 20 in-depth interviews from some experts in the field. But I agree with the pint of saturation. For the time being, I am going to use 10 in-depth interviews and then I will add if point of saturation is not reached.

I actually agree with the fact that data should continued to be sought until saturation is reached, but in the case of a mixed design, between 8-14 participants should be enough since the data from qualitative design is to support or disagree with quantitative design. Thank u

If your not triangulating, 10-12 will be enough. Therefore in qualitative in-depth study 10 respondents with no saturation.

According to Creswell & Poth (2018), there is not any specific answer. However, the number of participants depends on the qualitative research approach. According to Creswell, W. & Creswell, D. (2018), Narrative includes 1-2, phenomenology includes 3-10, the grounded theory includes 20-30. ethnography includes one single culture sharing group and the case study includes five to six cases.

References:

Creswell, J. W., & Poth, C. N. (2018). Qualitative Inquiry and Research Design: Choosing Among Five Approaches.

Creswell, W. J., & Creswell, D. J (2018). Research design: Qualitative, quantitative, and mixed methods approach. Sage publications.

Till the process of saturation

I think the Creswell and Poth recommendations are another a example of proposing "magic numbers." In particular, there is often good justification for conducting a single case study.

There are a variety of authors and numbers for the strategies of inquiry under umbrella of Qualitative research. Some of my colleagues argued for anything more than 30 as is done in QR. It depends a lot on your study context, connections and quality of referrals. Excuses and dishonoring of commitment and missed appointments in international endeavors are common

Additionally, you will want to consider the concept of “in-depth” that has bern mentioned. When conducting your interviews, is that a goal and how are you justifying the length of the interview or rapport developed as being in-depth? You could for instance interview people more than once, having a smaller number overall, but more in-depth, all with the focus of reaching saturation in responses across participants.

According to different books of qualitative research there are two conditions either to take maximum (n=40) or upto the saturation of the data.

It all depends on attaining saturation. However, one may go with at least twenty (20) participants in a major case study research. But, if the target-population is a "hard-to-get" group (such as gang members, prostitutes/sex workers, militants, etc), then one may go with a lesser number - maybe at least five (5) participants, as long as they have "in-depth" knowledge/information about the social problem under investigation.

The question is how the interview data support the model or other contribution of the study. Here is an editor's quote (the journal with IF 2.8) to one of my rejected papers:

"...we are not interested in thick data per se, but on the extent to which they are used to corroborate theory"

The saturation principle is relevant here.

About 10

Sample size guidelines suggested a range between 20 and 30 interviews to be adequate (Creswell, 1998). Interviewer and note taker agreed that thematic saturation, the point at which no new concepts emerge from subsequent interviews (Patton, 2002), was achieved following completion of 20 interviews.

Interviewing continued until we deemed data saturation to have been reached (the point at which no new themes were emerging). Researchers have proposed 30 as an approximate or working number of interviews at which one could expect to be reaching theoretical saturation when using a semi-structured interview approach (Morse 2000), although this can vary depending on the heterogeneity of respondents interviewed and complexity of the issues explored.