Each roster can have no more than 12 people dressed out to play.
There is no limit on the amount of guest players per team. During the course of the regular season teams are allowed to pick up players that are not on their roster in order to field a full team. In playoffs, players must be on the roster in order to play.
Guest players must have their own number on their shirt. The number must be large and permanent (sharpie on the back is acceptable). The number must be different from all other players on the team.
All guest players must sign the waiver prior to playing: https://fxasports.leagueapps.com/leagues/activities/311538-guest-player-waiver