Currently Yousician Piano has eight complete levels. Levels 9-15 are still under development, but you can find new content under Challenges tab every week.
If you're after more of a challenge, use the song search feature for content above level 8, to give yourself a serious workout. There's often a weekly challenge song level 6 and above as well.
Remember that if you have finished a path, you can still go back and work on one of the other two paths!