We used to do 10 in my league and switched to 9. 10 was too many. 9 is just right.
On friendly advice, I would remind the league that its 9 starts per matchup in regular season rather than per week. If you do a 2-week championship it’s 9 starts per week, not 18 per matchup.
Niv discusses it here: Head 2 Head - 2 week playoff - how many starts?