On mobile, you can add/remove favorite stations by tapping the "Follow" button located in the top right corner of the currently played station. You'll notice that the "Follow" button is now checked and has changed to "Following".
To remove as a favorite, tap the "Following" button to uncheck. The "Following" status will then return to "Follow".
You can also manage your followed stations in Settings > My Audio and selecting between "Follow some entities" or "Unfollow all".
Note: No limit to the number of stations you can follow.