According to an email I got from Google Adsense support they are not giving away more then 200 channels at the moment. Normally Google Adsense would send you an email and give you another 300 custom channels when your 200 was finished. That would give you a total of 500 custom and url channels in total.
When I first applied they said we will give you soon but later when I got another email they said sorry have stopped giving away more channels. But from what I saw other people still get more channels. It looks like Google Adsense like to keep people in dark. Even on the Google Support forum with very high status said they never heard of 500 channels.