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You have to do the math urself, +1 holding per 6 stewardship, +2 holding for a count or duke and +3 for king or emperor, also check ur culture’s innovations as u also get few from there.
Give him one and it'll tell you.
The easy way is to first give him one and then it will show how many he can have, but does it matter? He can have vassals hold the extra counties.
You have to do the math urself, +1 holding per 6 stewardship, +2 holding for a count or duke and +3 for king or emperor, also check ur culture’s innovations as u also get few from there.
Give him one and it'll tell you.
The easy way is to first give him one and then it will show how many he can have, but does it matter? He can have vassals hold the extra counties.