The Seattle expansion draft won’t happen until 2021 so the Flyers have a while before they need to worry about it.
But with the current season in limbo, decisions may start to be made soon. Moves made during this offseason will likely have effects on who teams will protect next summer.
The rules are the same as they were before. Teams can protect 11 players (seven forwards, three defensemen and one goalie). They also have the option to protect eight skaters and one goaltender.
Other rules are much more detailed and can be found here.