If a bigger crew/boat has to come in game it has to be a crew number that is a multiple of 24.
The server can only handle a maximum of 24 players.

Sloop -> 24players / 2 man crew = 12 boats max in server
Brigantine -> 24players / 3 man crew = 8 boats max in server
Galleon -> 24players / 4 man crew = 6 boats max in server

Next ship SHOULD be the next multiple of 24 and the bigger number is 6. However servers would feel utterly empty

24players / 6 man crew = 4 boats max in server

I love the idea of a bigger crew but Rare would need to increase the player cap first to at least 36 players to have the same experience as the minimum population possible in sea of thieves servers : 6 boats.