All you have to do to unlock Pro League is to go online and play 10 online matches in each sport. That means if you are looking to unlock Pro League for every sport, you will have to play at least 60 online matches, 10 for each sport.
How do you unlock ranked in Nintendo Switch Sports?
Instead, it is more so a matter of investing time into playing each of the games Nintendo Switch Sports has to offer. In order to unlock the Pro League for a given sport, the player simply has to play 10 online matches in that specific sport.