FED Rule 3-4-1 says:
Each team, when on defense, may be granted not more than three charged conferences during a seven-inning game, without penalty, to permit COACHES or the non-playing REPRESENTATIVES to confer with a defensive player or players.
So... more than one can enter the field and multiple conference can occur at the same time.
Humorously 3-4-3 says the conference ends when the COACH or non-playing REPRESENTATIVE crosses the foul line.
