Kingmaker - Player's Options 18
or
Kingmaker - Adventure Path 510
Step 7, Choose leaders
1st paragraph
"Every kingdom needs leaders, and in Kingmaker, the assumption is that those leaders include the PCs (though if the party has more than eight PCs, not all PCs will have leadership roles) (...)"
With this we know that we can have all 8 of the roles can be filled with PCs.
3rd paragraph
"Next, choose four leadership roles to invest"
The limit of 4 is to assign invested roles. You can have up to 8 invested roles, but then you have to choose the right NPC to each kingdom role, as them can auto invest when chosen.
So now you have to choose if you want the extra proficiency for having NPC investing in a kingdom role or extra leadership actions for having PCs.
