how do you plan to analyse the data?
If you chose a quantitative approach, there are certain rules on how to select a sample size representative of a well-defined population. Your sample needs to be diverse enough to facilitate a meaningful analysis.
I have very limited experience in qualitative research designs, but I am given to understand that you might be able to draw conclusions from case studies. In this case, your results cannot be generalised (findings will be about these participants in these conditions). From the description of your research design, I think that grounded theory might be the best approach. In grounded theory, the sample size is as big as it is needed to achieve saturation of information.
Good luck