It's not just a simple number press (although you can set it up for some skills), but it's key combinations. And building up some combos is not just LMB, 1,2, LMB, 3,4,5 but rather:
[SHIFT]+[SPACEBAR] followed by
[F] then
[SHIFT] + [F] followed by
[SHIFT] + (S) followed by
(S)+[F] followed by
[RMB] then
(S)+[E] and
[SHIFT]+[LPM+PPM] finishing with
[LMB].