I know someone who has 15. They told me they can't pay any attention to their teams on Sunday/Monday - the conflicts are too plentiful & too maddening.
They decided they could either let FF ruin football for them & drive them insane, or simply check for W's, L's and injuries on Monday/Tuesday.
I was in 3 redraft & 1 dynasty and have since pared down to 1 RD, 1 Dynasty. The RD is IDP, so they're sufficiently different animals, and even then I still get conflicts from time to time where I'm cheering and booing the same play at the same time.
I vowed to not expand beyond these 2 leagues (both with people I know locally) and have stuck to it, despite numerous invites every year. It's just better for my mental health, which is already borderline without FF.