Is there a maximum number of attendees?
On the Free plan, you cannot exceed 10 simultaneous users.
On a paid plan, you can safely plan for more than 1,000 people on a single map. What you pay for is the capacity you subscribe to, not a technical ceiling. Plan the map to match: it has to be big enough for everyone to be on it at once, with room to walk.
If you are organizing a long-lasting event, remember that the total number of attendees and the number of people connected at the same time are two different figures. It is the second one your plan has to cover.
