There have probably been a handful of threads where people have discussed which keys they use, and this very old thread has some good discussion.

With that said, I currently use:

[Movement] E,S,D,F
[Left/Keyboard] 1,2,3,4,5,,Q,W,R,T,A,F,Z,X,C,V
[Right/Mouse] 6,7,8,9,0,-,=,H,UP,DOWN,LEFT,RIGHT,MWU,MWD,MOUSE3

All combined with single or multiple modifiers, as well as on-screen clickable buttons, created with ISBoxer.

Whenever I finally get back around to dedicating time to multiboxing BFA (we're close now), I'll probably be looking to expand upon that list if I begin pushing high-level Mythics (not related to Classic, but I thought I'd give a reason).