Sea of Thieves will launch with a maximum crew size (and therefore party size) of four.
Lead designer Mike Chapman confirmed to IGN that the party size from the game's beta would continue into the full game, saying that the key element for Rare was in how larger groups would (or wouldn't) communicate:
"The biggest ship was designed for four," Chapman explained when asked why that became the maximum. "We've looked at feedback and of course there's people who want an 8-player ship, a 10-player ship. The thinking there, as with everything in this game, is really intentful.
"If the two of us were to go out to the pub together with two other friends, you've got that intimate relationship, you're all getting on together. If it becomes six or eight people, you start getting people splintering off and it's really hard to communicate - four seems like the magic number."