Some interesting calculations and assumptions made here from the achievements which have been logged?
Basically, 0.23% of people who've played the game have gone on to become pirate legends. That's out of roughly four million players, as announced by Rare during E3. That said, the redditor then goes on to only include those players who have earned the achievement for leaving port, which rather surprisingly is just shy of two-thirds, and of those players, 0.36% of players have gone on to become a pirate legend.
The referenced reddit post is here - I didn't check the maths!
https://www.reddit.com/r/Seaofthieves/comments/8xg9hr/there_are_more_pirate_legends_around_than_you/
