As part of the CBA proposal on transactions, MLB wants to reduce the size of the Domestic Reserve List to 150. This list came about as part of the restructuring of the minor leagues:

The Domestic Reserve List was implemented as part of the changes. The league, a league source said, regarded 180 players as a high number owed to the new minor league system and the missed year of development due to COVID-19. Teams previously were not limited in the number of players they could have under minor league contracts. Five teams presently have more than 180 minor leaguers on their Domestic Reserve List, according to the source. (Teams can have up to 190 players on it during the offseason.) Two teams, the source said, have fewer than 150.

The potential roster trimming was part of a 28-point package on transactions that included a proposal to limit the number of options — or times a player can be returned from the major leagues to the minor leagues in one season — to five, according to sources. Currently, teams hold unlimited options on qualifying players within a single season, allowing the shuttling of some between Triple-A and the major leagues.

ESPN.com

I thought it was possible for someone to play for a minor league team without being controlled by the major league club, but the uniform minor league contract would appear to deny that. Note that with fewer minor league teams, fewer players is somewhat inevitable.

Since there is no floor on the current number, teams are perfectly capable of driving the reserve list down to 150, and some already did. The main positive of the lower number would be increasing competition, since it would be a bit tougher for rich teams to control more good players. In reality, I don’t see the 150th player on any team being very good.

I’m glad the union is saying no to this. The MLBPA threw amateur players under the bus in previous agreements, thinking the money saved on the low end would lead to more money for major league players. That did not happen.

There is also an International reserve list based on the number of Dominican Summer League teams.