data collection and analysis are labor intensive.
It is important to use the labor efficiently by
thinking out exactly what your sampling strategy
will be. The good news is that sample sizes are
small compared to sample sizes for data that will
be analyzed by quantitative techniques. With careful
sampling and equally careful collection techniques,
a surprisingly small number of interviews, narratives
or focus groups can yield the data to answer your
general rule in qualitative research is that you continue
to sample until you are not getting any new information or
are no longer gaining new insights.
here for a table summarizing sampling techniques and protocols.