The question of 'how many qualitative research sessions are needed for a study' comes up a lot in my work. So I put this guide and structure together to explain my approach to it.

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It makes sense to combine research and testing in sessions.

If you are pre Discovery phase and don’t know anything about your users then you need to have included at least 15 participants in your study to adequately gather enough insight to understand the different user groups. From there you can decide if you want to focus on one user group for MVP or understand how distinct users respond to your designs.
 

The Discovery research with 15 participants can be done all in one go, or it is also possible to do this over 3 separate studies to gain traction and move fast with designs, if there are concepts ready to usability test.
 

Post Discovery if you want to focus on just one user group to usability test with, the best practice is to learn quickly from regular smaller studies with 5 participants at a time. The first round will not answer all of the research and usability questions you have about particular software. But it allows you to learn quickly about 80% of the problems, make changes, iterate and learn further, in a much quicker way. Running bigger studies all at once at the beginning slows you down.
 

How to approach this will depend on whether distinct groups of users have been defined, or if they mainly fit one user group
 

If there is one cohort then it makes sense to quickly and iteratively test with 5 participants at a time

If there are different sets of user groups then testing should be done with 3 participants per user group – e.g 9 participants at a time (if there are three user groups).
 

See for reference: Why You Only Need to Test with 5 Users (nngroup.com)