It's 47 including the host its 48 (with a maximum of 40 on track).

Why not? Whole physics calculation, replays etcetc it all has their limitations. Not even mentioning grid slots and pit garages although in open environment something could be done about that (I guess).

Name a different >racing< sim which has more race/spectator slots? There you have your answer.

Looks like it is 12 for demo users, 15 spectators + the host ?

Yes but maybe he considers buying a license and wondering what happens in this situation.