Hi Monika and great project. This is A LOT of interviews. Often a study within one group will reach thematic saturation after about 20 interviews. If you are interviewing multiple groups, it may be reasonable to do 10-20 per group, but keep in mind this will be a lot of data, and you might want to limit your study initially to the perspectives of one group. There will also be transcription and translation issues to think about - you will be publishing in English? So you will have to consider will bilingual coders do the analysis as well?
re: generalizability, i'd say its similar to quantitative research
the interview guide should be very open-ended questions - but be clear about your research question to define the scope to participants