This is a question that I think all qualitative researchers struggle with - how many interviews do you need to produce adequate, meaningful evidence? As a transcription company, we've dealt with projects with five interviews, and others with hundreds. There are certainly a lot of questions to consider, and Sarah Baker and Rosalind Edwards' very useful working paper aims to address these. Rosalind was interviewed for an NCRM podcast about this paper, which we've transcribed below for you.
At Transcription Centre we specialise in transcription of interviews, focus groups and speeches for academics and market researchers primarily in the UK.