I think 8 seemed right at the peak. Though now with how high prices are I have to wonder if that many are sustainable.
Uncanny
X-Men
X-Factor
X-Force
Excalibur
Generation X
Wolverine
Cable
Though I'm not a fan of the solos so I could do without but reality is we'll get a Wolverine solo eventually.
Just have one X-Men book. Add in an Academy X book. Maybe a bad guys team since there's so many on Krakoa. Or even an Illuminati type book featuring the Council. Keep each premise unique and the casts mostly separate.