Tried and tested with 8 players would surely work. Usually that would give you an advantage of close to 30%+ in finals, unless drawn with similar manipulating teams or scout buying players (Last season a league opponent did buy 8 scout players to fight for the title with me and he still lost

, so this does happen even when it is rare).
And if one has 5 x 1* players, even 9 players would work, as I have done by bringing 9 players of average quality of 120%. Best opponent is ‘only’ 95% in CL final and 83% in Cup final when both started their season around 72-73%, when mine starts the season around 110% with 2 additions of 90% players to complete the first-11 and now is 121% after strengthening.
Will update further on 10 players brought forward next season to see if it works as well.