Whatever number of snakes with the individual snakes that make you happy and your life stress-free imo!
Everyone's number is different. I don't think I personally want to go above 8-9 total, and I never have gone above 9-10 at one time even though I've owned, I think 13 in total at different times. I want to devote my other monthly spending and time to my dog, other hobbies that make me happy, and for work.
I'm sure for some people, having a couple snakes with pleasant personalities is the most rewarding. Maybe the snakes are high-maintenance or grow large. Some people would rather collect a larger number of color morphs of the same species, but smaller and lower maintenance, and find that rewarding. I've found so far that I'd like to keep a variety of species and experience their interesting and/or personable temperament and intelligence, and that's most rewarding to me. It's all on you!