There is no strict limit, but three characters maximum is recommended by player.

For this event the rule regarding canons are a little bit more lax and you can play characters from your cast so long as you don't playercest (play against your own characters). However, if you plan on continuing to play them after the event, there is no guarantee their application will be accepted (eg, if you play Luke Skywalker, you may be able to play Emperor Palpatine during the event but you won't be able to app him properly afterward).