Are you asking how the max party size works?
From what I can tell just based on my own stats the math works out like this:
I am playing on normal so it may work out differently for other difficulties.
Base: 10 (assumed)
Charisma: 16
Leadership: 5
Renown: 1458
Party Size: 258
258 -10 (base) - 16 (CHA) - 25 (Leadership multiplied by 5) = 207
1458X = 207
X = ~1/7 (0.142 vs. 0.143)
So, the base and charisma are just added, leadership is multiplied by 5, and renown divided by 7 but truncated (I think) to whole numbers.