Some days (more in the begin of season) i used 12 (bench included so for 18 players) but playing all comps (league/cl/cup),and some days only 4 and only for the one who almost never play because the 11 who played still had a high moral..
But,i won 25 games and 1 draw in league,maybe because of that?
!?More wins = less boosts used?!

I set only 6 players at the top of moral,never all 11 and i do it 1on1,never from the main page,if you do it from the main page (like your picture) the game takes you 1 more then you really need.