Crew Size Limitations

Charisma and Leadership
 
A good captain can lead a large crew into battle and inspire others to join his cause. Charisma and leadership is required for a good captain, arguably. Luffy showed natural charisma by winning over the most beautiful woman in the world, Whitebeard showed great leadership in his first and last arc against the marines. Leadership/Charisma will control crew size because of these key facts about one piece. Also because I don't want a guy with zero leadership controlling an army.

0% cannot be a captain.
10% cannot form crews but can become captain if a captain should die, can be a first mate.
20% can form crews containing four people or less.
30% can have crews of six or less.
40% can have crews of eight or less, user receives 10 npc's with 10 in all natural stats and 20 in two stats (minus devil fruit control) for their crew.
50% can have crews of fifteen or less.
60% can have crews of twenty or less.
70% can have a crew of twenty five or less. Receives 50 npc's with 10 in all natural stats except devil fruit control, they get two stats with 20 in them.
80% can have a crew of thirty or less.
90% can have up to ten divisions in their crew consisting of no less than five members each excluding npc's
100% no limit to crew size, receives 100 npc's with 10 in all stats naturally and 20 in four of them.

Note: The PC's are the 'crew' in this explanation. An example is 40%. In 40% you can have a crew of 8 or less, the captain receives 10npc's separately. The crew of 8 are all PC's, the NPC's are a number all together. Also note that only captains get these bonuses, not the first mate or anyone else in a crew.