This is fairly typical for a newly opened scenario. You'll just have to be patient and keep refreshing the games page, waiting for an opening. There were some spots opening occasionally on Modern Times #2 after a week or so. Then, of course, Air Travel Boom opened and a number of people decided to quit MT2, who I assume were players that didn't have a good situation in MT2 and were hoping for a better chance in the new game. Regardless, I assume that the same will happen for Air Travel Boom after it has been open for a week or two, openings one by one. 500 players is already stretching the limit for reasonable competition in many markets, so I don't think you're likely to see more spots added to the maximum. Just keep watching and waiting.