As many as you want! There is no limit.
Theoretically, you could add every stream in the world, though that would get quite expensive. There is a small monthly fee for each station added to your account.
To add stations, just increase the number of stations monitored in your account. There must be an open slot available to add a new station. After we confirm the increase, you can choose any station already monitored or add a new station through the Add Station Request feature.