One of my students shared this paper with me. It talks about when you should expect to see saturation in a focus group study. It uses some hard date to answer this question.
Hennink MM, Kaiser BN, Weber MB. What Influences Saturation? Estimating Sample Sizes in Focus Group Research. Qual Health Res. 2019;29(10):1483-1496. doi:10.1177/1049732318821692. Available in html format or pdf format.