Welcome to the club RW. I agree with Neil, tricky one to answer and hopefully you will get a lot of answers here. There will be no right answer and I'll give you a couple of my thoughts and experience.
Prior to my current set up I had 5.1 R/L front floor standers with smaller center and R/L surround. My current set up are all bookshelf and one sub, 5.1. My new set up in a few weeks will be 5.1 with 3 subs, R/L front floors, two rear surrounds and one center with 4 ceilings, Atmos.
If you are like me, a gearhead sometimes it's easy to forget the goal and that is to listen to music in a comfortable space.
Your sitting and listening space, how you create that has a lot to do with your experience. Walls that reflect, absorb, etc will all affect your speaker source. I have used different speakers over the years, but the ones that have captured my heart and soul have been the B&W and Kplisch brands.
I look at all of this as a hobby, so try not to go too fast, adding little pieces, waiting awhile before a next piece, is a good way to go. Like a home train hobbyist, the fun is building the track and town, the watching the train go round is the fruit of your passion.
This site from Wendy Carlos is very good to study and will help you on your personal journey.
http://www.wendycarlos.com/gosurround.html