I'm pretty sure they will end up with more than 10 crewmates. They have 10 now with Jinbei (not counting Vivi, Gaimon, Kraken and all others) And they are only halfway the journey. They are bound sooner or later to pick someone up. For a change in dynamic the need a refreshing air on the ship once in a while. Besides Luffy said 10 crewmates at the start of OP. Oda admitted that he never thought his manga would run for 10 years. Let alone be at the halfway point.