I do not know about this specific ship but on most ships you will have to ask your senior waiter to get you either a booster or high-chair. Once you make that initial request, it should be waiting at your table for you each evening.
Tables come in all sizes- 8 being the most common.
If you have a TA be sure she/he lets the ship know ahead of time that you will need a highchair (and crib if need be). Our TA told us she would do this for us just to be sure they had enough on board.