Hi Ray, it all depends if you want to really be immersed in the sim -one big point to consider is - do you want to use your keyboard while racing, or would you rather be 'in the car', using the dash controls - If so, don't underestimate how many you will need. First, I fitted a pro race display to my G27 wheel, this display includes 8 buttons and two rotary encoders. Then I built a DIY button box, with 23 buttons and 3 rotary encoders.
I also use XPadder software, which allows me to use the same buttons for several functions, I highly recommend it for about a fiver. I somehow manage to use them all when sim racing, and I wouldn't be without them. It makes a massive difference to the immersion, to be able to drive and access all buttons without stretching to reach my keyboard. And I could still find more to add lol.
It's a bit too obvious to list them really, but they are based on three areas:
1) in-cockpit controls - used in-race - these include pause, ignition, start, wipers, headlights, pit request menu selections, look left and right, all the things that you need close at hand and in the most convenient places while driving. Most of these are on the wheel;
2) other car/pit controls - seat adjustment, HUD, mirrors, 'skip formation lap', camera, replay - all those controls that can be used while paused, or sitting in the pits, or on an out-lap;
3) other computer controls - AltTab, Space, Ctrl, Shift, Esc, Ent - all the controls that enable me to leave my keyboard alone, keeping me immersed inside the sim .
It's not the greatest looking setup, as the button box is basically a rectangular aluminium box with all the buttons and rotarys covering the front and top surfaces. But it is all right where I need it, and it works. At the moment, the only time I have to reach for the keyboard is when I save a setup and have to type a name in, so I probably need to use x-padder to make a macro for that

With the button box, G27, and ProRace, I have over 40 buttons and 5 rotary encoders, and I can easily increase this number using x-padder.
Anyway I hope there's some helpful hints here.