Why is it that even a yacht of 80 meters can carry only 12 guests in some cases?

Because of the regulations you are only allowed to carry 12 guests in private mode.

Often yachts can carry more than 12 guests. In commercial operation though, unless the yacht is certified to carry more than 12 passengers, the yacht is limited to 12 passengers. This does not mean that the yacht cannot carry more guests onboard, but it will have to do so in private mode and this needs to be agreed with by the Flag State and often the States of the territorial waters where the vessel wants to operate.

Hope this helps make it clear!