I’ve found the oft-repeated claim of “you only need five users for usability testing” largely true when testing something relatively straightforward and when you really only care about a handful of things. A majority of my usability studies have five to 10 users. Interviews can vary a little more because of the wider variety of interview studies, but most of my own have had sample sizes of seven to 15.
You can’t really know when you’ll have enough data until you start collecting that data.
How many users are needed to reach saturation will depend a lot on how many research questions there are; how complicated or nuanced those questions are; how much your team already knows about the problem space; and how many sampling criteria there are for the study, just to name a few.