Plus, keep in mind that even if you have a room that holds say three for example, and you are only a party of two now, that does not guarantee availability later on. As was mentioned above, the ship fills to capacity according to ages under 18, the dining rooms, and most importantly the lifeboats. So, if their quotas for lifeboat drill areas are full later on, then you may not be able to add another occupant to your room even though there is a bed in there, for example.
Another good point is that the earlier you book, the better the rates. Better to do the deposit now, as was mentioned, and cancel prior to the 60 day window if necessary. Good luck!