Major League Baseball announced the rule changes that will be in place for its 2020 Championship Season:

Three-Batter Minimum:

The Official Baseball Rules have been amended to require the starting or any relief pitcher to pitch to a minimum of three batters. The three-batter minimum will become effective in 2020 Spring Training beginning on Thursday, March 12th.

Active Roster Limits:

Rosters through August 31st and Postseason: Active Roster limits from Opening Day through August 31st and including Postseason games shall be increased from 25 to 26.  In addition, Clubs will be permitted to carry a maximum of 13 pitchers from Opening Day through August 31st (plus Postseason games).

September Rosters:

From September 1st through the end of the Championship Season (including any tiebreaker games), all Clubs must carry 28 players on the Active Roster. In addition, Clubs will be permitted to carry a maximum of 14 pitchers during this period.

Two-Way Player Designation:

Players who qualify as “Two-Way Players” may appear as pitchers during a game without counting toward a Clubs’ pitcher limitations. A player will qualify as a “Two-Way Player” only if he accrues both: (i) at least 20 Major League innings pitched; and (ii) at least 20 Major League games started (as a position player or designated hitter) with at least three plate appearances in each of those games, in either the current Championship Season or the prior Championship Season (for 2020 only, this will include 2019 as well as 2018).  The Club must designate that player as a “Two-Way Player” in advance of that game. Once a Club designates a qualified “Two-Way Player” that designation will remain in effect, and cannot change, for the remainder of that Championship Season and Postseason.

Position Players Pitching:

Any player may appear as a pitcher following the 9th inning of an extra inning game, or in any game in which his team is losing or winning by more than six runs when the player enters as a pitcher.

Extra Player Rule:

The previous “26th player rule” will be replaced with the “27th player rule” for all applicable Championship Season games prior to September 1st. The 27th player shall not count toward any pitcher roster limits described above.  Thus, a Club may designate 14 pitchers in games under circumstances where the Major League Rules would permit a 27th Active player.

Injured List Reinstatements and Option Period for Pitchers:

Clubs may not reinstate pitchers or Two-Way Players from the Injured List until 15 days have elapsed from the date of the initial placement for such injury – an increase from 10 days.  In addition, the option period for pitchers will be lengthened from 10 days to 15 days.

Reduction in Challenge Time:

Managers will now have up to 20 seconds to challenge a play instead of 30.

Source: https://www.mister-baseball.com/mlb-announces-rule-2020-season/