It's really a factor of how much ballroom space they have, because that directly affects how many people you can put in that space due to fire codes and whatnot. When Evo (big fighting game tournament) was at the Rio a few years back, there must have been at least 2,500 people registered, due to size of the entrant pools in some of the games that year. Assuming SE is renting the entire ballroom space, I could see more than 1,800 tickets being sold. But all we can really do is speculate.