The right number
is in the paperwork
The Italian Recreational Boating Code (Legislative Decree 171 of 18 July 2005) devotes a whole article, number 34, to the maximum number of people that may be carried. For CE-marked natanti the number is documented by the builder's plate and the owner's manual (paragraph 3, letter a). For natanti without CE marking you look, if they are type-approved, at the copy of the type-approval certificate and the builder's declaration of conformity; if they are not type-approved, the implementing regulation of the Code applies (letter b).
For registered craft, the imbarcazioni and navi da diporto, the number is entered on the navigation licence by the authority that issues it, on the basis of the technical documentation (paragraph 1). If a boat has more than one design category, the maximum number is the one set by the builder for the category that matches the navigation actually under way (paragraph 2).
What a natante is, art. 3 of the Code explains: a craft with a hull length of 10 metres or less, measured according to the UNI EN ISO 8666 standard, which is not the same as the commercial length overall. The largest size in the NAVY range, the NAVY 9.50 Cabinato, is 9.80 metres long even overall.