There is no maximum limit, but if in the end for example one team has 7 members as opposed to all the other teams having 4, then the team will have to split to obtain the closest numerical equality amongst all team sizes.
However, duelists signing up on the orphan registration thread will be added to teams that have fewer members than the largest team, so as to maintain as close numerical equity among the team sizes as we can. If there are not enough orphans to make all team sizes equal, either the orphans will be made into teams of their own or they will be randomly added to any registered team.
For the Team stages, regardless of the number of duelists on one's team, the team can only assign a certain number of duelists to each rank for a particular stage, as follows:
Champion: 1 per team
Dan: 2 per team
Kyu: 3 per team
Teams are not required to field the maximum for each rank; if needed, teams can even leave an entire rank unfilled, though this is not recommended for the Champion rank.
Team stages are not the only time teams are ranked against each other; every week, the teams are ranked by the total points (TP + SP + FP + MP) of each team's top 3 duelists in total points. The members of the top ranked team at the end of the Tour will be honored in the awards ceremony.