If you choose the freemium plan, you cannot exceed 15 simultaneous users.
If you choose a premium plan, technically you can safely plan to have +1000 attendees on a single map. Be aware that you will have to design or pick up a map big enough to welcome all your users.
The largest event up to date was the remote "Chaos Computer Club" event, that was organized on a modified version of WorkAdventure to create "parallel universes" on the fly. It welcomed +10000 attendees!
If you are an event organizer, do not forget there is a difference between the total number of attendees and the number of simultaneous users connecting.