In qualitative research, the notion of saturation is largely a judgment call: It is the point at which additional informants are not yielding new information about the phenomenon under investigation.
Niccolai et al. (2016) sought saturation during their analysis, but also state (p. 843) that:
An a priori sample size of 30 to 40 was selected based on recommendations for qualitative studies of this nature… and the anticipated complexity and desired level of depth for our research questions.