According to the FAQ-
How many tickets can I buy for each event?
Most events have a limit of two (2) tickets per person, but some events have different limits. For example, True Dungeon allows you to buy all the tickets for one event so you and all your friends can play at the same time.
How can I tell what the limit is for a particular event? It seems reasonable to assume that most 4 person events have the 2 ticket limit but what about a 6 person event? Or more? I would like to know this ahead of time before getting screwed over because I tried to buy too many tickets for a given event.