Recker and Carson are currently on the 40-man roster, although den Dekker is not. There are 39 players on the 40-man roster.
Carson can take Harvey's roster spot, but I have to wonder if the other transactions involve more than just roster swaps with Triple-A Las Vegas.
The Mets need to add den Dekker to the 40-man roster before this winter to protect him from Rule 5 draft in December. With Jeremy Hefner, Jenrry Mejia and Matt Harvey -- and perhaps both Josh Edgin and Bobby Parnell -- all lost for the season, the Mets can free up one active roster spot and five more 40-man spots by placing each of them on the 60-day disabled list. That gives them flexibility to add den Dekker, as well as players like Jacob deGrom, Cory Vaughn, Allan Dykstra, Darrell Ceciliani and Jeff Walters, (all of whom also need to be added to the 40-man roster in the short-term); or they could also add Tim Byrdak, who can help fortify the bullpen. That doesn't mean any of them will be promoted when rosters expand, however.