As of today, it is allowed to gather up to 1000 people in the open. A large number of people can attend sports competitions with a distance of at least one meter. You can view and download the order here

https://www.mos.gov.rs/vest/naredba-o-zabrani-okupljanja-u-republici-srbiji-na-javnim-mestima-u-zatvorenom-i-otvorenom-prostoru