Quite a number of MLB ballparks will be open to small numbers of fans to start the 2021 season. For example:
The Kansas City Royals have announced fans will be allowed to attend games at Kauffman Stadium starting with the season opener. initially, pod-style seating will allow 10,000 fans per game. The announcement states a goal "to expand the capacity monthly." #Royals
— Lynn Worthy (@LWorthySports) March 4, 2021
Busch Stadium in St. Louis will have up to 15,000 fans permitted, and the recent reopening of the state of Texas has raised fan hopes for the Texas Rangers:
One source indicated hope that the Rangers will be able to allow full occupancy for the start of the season.