Unlike quantitative research, in the qualitative research the sample size is not calculated before conducting the study. The most important factor is data saturation in which there is no new idea or code and you gathered all data according to the aim of the study. So you may reach to data saturation with small size or large size. For more information, you can read the followings:

Sample Size in Qualitative Interview Studies:http://journals.sagepub.com/doi/abs/10.1177/1049732315617444?journalCode=qhra

Sample Size and Saturation in PhD Studies Using Qualitative Interviews:

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

Mansour Ghafourifard gave you a good answer already. The sample size depends on the condition you are studying. How many patients are in a certain treatment at a special time. 

Dear Qureshi,

You have brought up an interesting topic for all researchers who are interested in employing qualitative methods in their studies.

In qualitative research, since researchers invest a great deal of time and resources to accrue 'quality', thus, thick descriptions of phenomenon studied using the key and effective qualitative data collection instruments such as interviews and observations, it is advised by many scholars in research that small and manageable sample size must be selected. Of course, the situations in phenomena studied by qualitative researchers vary and must dictate the selection of the appropriate sample size. Yet, to reduce the subjectivity and bias that is often seen as weaknesses in qualitative studies conducted, I will advise that you also try including some mathematical method in the selection of the sample. For instance, Leedy and Ormrod (2010) suggested that a population size of 500 must have a sample percentage of 50% to qualify as a true representative sample of the population. Thus, their premise could be a starting point for the selection of the sample size but you must be cautious of the variance in circumstances surrounding the study of the phenomenon. All the best.

Hi, would you please share the sample size of the qualitative research and how did you calculate it?

Agreed with the answers provided by previous researchers / scholars.  Sample size in qualitative research also depending on your research objective.  E.g., if your objective is to extract maximum insights until data saturation from an interviewee who is the United Nations General Secretary, perhaps 1 sample is good enough.  But if you try to extract insights about certain leadership traits among entrepreneurs, perhaps you might want to interview numerous entrepreneurs until data collected are saturated.

The following sources 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.
• Marshall, B., Cardon, P., Poddar, A. and Fontenot, R. (2013). Does sample size matter in qualitative research?: A review of qualitative interviews in IS research. Journal of Computer Information Systems, 54, 1, pp. 11-22.
• Sandelowski, M. (1995). Sample size in Qualitative Research. Research in Nursing and Health, 18, 2, pp. 179-183.
• Saunders, M., Thornhill, A. and Lewis, P. (2003). Selecting samples. In: Saunders, M., Thornhill, A. and Lewis, P. (eds.) Research Methods for Business Students. 3rd ed. England: Pearson Education Limited, pp. 150-181.  

https://www.researchgate.net/post/How_many_interviews_are_enough_in_qualitative_research_case_study_studies/1 

It also depends on your data collecting methods and techniques.

For exemple if you use life story interviews you will probably not need more then 30 participants because your data collected this way will be large and detailed enough to draw some conclusions. But if you use drawings, for example, you will probably need more participants.

Of course you will decide exactly how many as your categories are saturating. You could stop at 10 or at 100 depending also on the theoretical sampling criteria (on the size of the list of characteristics discovered to be relevant for sampling).

Thank you all for the suggestions. I am obliged.

I have one more question, what if we use a mixed approach. A mix of qualitative approach and quantitative approach for a similar project?

The following papers could further help: 

• Collins, K. M. T., Onwuegbuzie, A. J. and Jiao, Q. G. (2007). A mixed methods investigation of mixed methods sampling designs in social and health science research. Journal of mixed methods research, 1, 3, pp. 267-294.
• Onwuegbuzie, A. J. and Collins , K. M. T. (2007). A Typology of Mixed Methods Sampling Designs in Social Science Research. The Qualitative Report 12, 2, pp. 281-316
• Onwuegbuzie, A. J. and Leech, N. L. (2004). Enhancing the Interpretation of "Significant" Findings: The Role of Mixed Methods Research. The Qualitative Report, 9, 4, pp. 770-792.
• Teddlie, C. and Yu, F. (2007). Mixed Methods Sampling: A Typology With Examples. Journal of Mixed Methods Research, 1, 1, pp. 77-100.

Leadership for successful mergers & acquisitions and if consideration of the human side improves results.