As a Free iHeartRadio user, you can add as many songs to your My Playlist as you would like.
As an iHeartRadio Plus subscriber, you can save up to 40 songs in My Playlist. As you add new songs older tracks will become inaccessible.
With iHeartRadio All Access there is no limit to the number of songs you can add to your playlists or the amount of playlists you can create.