The optimal number of people at an event depends on the number of ambassadors. For an average meetup with two organizers, ~25 people seems to be a good number.
If you're getting more people than you can handle, consider adding more ambassadors or splitting the meetup into two regional meetups. You can also use meetup tools to cap the number of attendees.
It takes time to grow your event, so the first two or three meetups may have low attendance.