[mc32]

Often you can find that they will charge $xx per additional person, if you go over the listed figure (if you choose to declare the overage).

[late2part]

More often they will declare >N persons a 'fire safety hazard' and refuse for your safety and security. And to help them make more revenue.

[itsmartapuntocm]

Cruise ships do this too, even if the extra person is an infant.