The answer is of course: it depends. But this NCRM Methods Review paper gathers and reviews responses to the question of ‘how many’ from 14  renowned social scientists and 5 early career researchers.  Based on the abstract: in considering what ‘it depends upon’ however, the responses will offer guidance on the epistemological, methodological and practical issues to take into account when conducting  research projects. This includes advice about assessing research aims and objectives, validity within epistemic communities and available time and resources.

I found the report through @Lalalalinder, you can download it here.