Having read and believed that God knew from before the Creation who would come to Christ, it seems to me that he knows exactly who (therefore how many) of the Gentiles belong to Jesus. Therefore, the full number has less to do with a full complement (as with the ship’s crew metaphor) and much more to do with God waiting until every one who is known to be part of the Church (and is named and counted among the saved) is aboard.
This is one of the dangers in taking an analogy too far. Of course there’s both a full number for the Church and that number has to be composed of people God knew would be saved before He laid the foundations of Earth. One of the important things about Romans 11:25 is that it takes us away from our fixation on a particular date and brings us back to helping achieve the required number for the Church by fulfilling the Great commission.