Adding or removing passengers can usually be done up until the final payment date without any penalty. The exception would be with a non-refundable deposit. When you cancel the passenger in a room with a non-refundable deposit, you will lose the deposit allocated for that passenger.
It is possible that the room you have reserved will not accommodate the new number of guests you would like. You may have to switch rooms or even categories to allow this. There is also the case when there is enough beds in your stateroom for more guests, however the ship, category or area of the ship has filled to capacity and you will not be permitted to add anyone. If your room will have a refundable deposit, it is always best to add the most number of guests you may have and reduce down later when you know for certain someone won't be going.
This is hard for some to understand if you have enough beds why you cannot add someone to your room. A ship is much different than a hotel in that there is limited space for people and services. The crew onboard can only handle a certain maximum number of passengers. Additionally, if there are problems at sea and you have to be evacuated, there must be enough space in each lifeboat.
The other reason to add the maximum number of guests in the beginning is for pricing reasons. If you are able to add guests later, it usually is at the prevailing rate at the time they are added. If the prices have gone up, you will pay more for the additional guests than what you would have paid from the start.