A match is played 32 against 32 against 32, so a team aims for a roster of 32.
Under 32 is a warning, never a block. It lowers the Roster fill part of the team’s reputation, but only once the team has played its first 10 rated matches.
Every player on a roster needs a SteamID64. A CSV needs the steam_id column filled on every row; a row without a valid one is rejected and not imported.
The quickest way to fill it is an invite link: make one on the Roster page, with a limit on uses and days if you like, and share it with the whole team. Inviting one person by name still works.
A Team Leader can make up to 12 team roles, such as Pilot or Medic, and the Leader or a Team Rep gives each person up to 3; they are labels on the team page and change nothing about what anyone can do.