Earlier today, the Mets added six minor league players—Noah Syndergaard, Cory Mazzoni, Hansel Robles, Gabriel Ynoa, Akeel Morris, and Jack Leathersich—to their 40-man roster and removed Jeff Walters from it to settle in at exactly forty players. Players are that roster are protected from the Rule 5 draft, which will take place on December 11 and allows teams with open spots on their 40-man rosters to acquire eligible, unprotected players for a nominal cost—so long as that team is willing and able to keep the player on its 25-man roster for the entirety of next season.
Here's the Mets' current 40-man roster in its entirety.
Matt den Dekker
Eric Young Jr.
Moving forward, the Mets cannot protect any additional eligible players from the Rule 5 draft. If the Mets acquire a player on a major league contract between now and the draft, they will have to remove a player from the current 40-man roster. Major League Baseball's non-tender deadline, by which teams must decide whether or not to give contracts to their arbitration-eligible players, will give them the opportunity to do just that if they would like. And any player can of course be removed from the 40-man roster, but the Mets would risk losing any such player to another team as a result.