A maximum of 12.
A private yacht charter is basically renting a boat for a certain number of hours, typically 3 to 5, for you and your guests to enjoy taking a day trip on the ocean. A captain is included for safety, but you have the run of the yacht.
Sure, bring whatever you like. The charter will include an ice chest onboard and some waters.
Dress according to the weather, and with layers. Note: some yacht owners ask that you take off Your shoes.