Yes—my understanding is that they hold maybe half to book onboard. Many threads on CC discussing ease of booking specialty restaurants onboard because people cancel and change all the time, especially once the CT menus are known. We always do this with no problem. You can check as soon as you board through the Voyager app, the TV in your room, through the scheduler at the WC (port side desk I think), or at the restaurants when they open in the evening.