Question: Having a full time non-musical job, I can only practice on the piano only as much as I can after work. At this point, as the momentum rises and my practice time has increased, I become enthusiastic about practicing multiple piano pieces because I like and get challenged by a lot of them (composers' masterpieces and advanced piano arrangements).
Should there be a limit on the number of piano pieces I can work on? Can you share a good musical principle in view of this decision?
Albert's reply: Ironically, one of the things that will accelerate your ability to learn and retain any given piece of music is practicing a lot of them. Learning and memory are cumulative. Thus, the more you learn, the more any given piece of music will make sense to you and the greater your capacity for learning music will become.
However, this only holds if you apply the proper learning methods to everything you practice. (See the article How to Learn Music for an outline of a methodology for practicing piano.)
For each piece you start, it's vital to lay the foundation for learning. This includes analyzing the score as well as writing in fingerings. Everything—even pieces you never intend to perform—must be practiced with utter seriousness, which eliminates any sloppiness or inaccuracies.
This method trains your mind to concentrate, as well as to absorb musical material. If and when you do return to a given piece, you'll find it vastly easier to learn since you will have already laid the proper foundation in your mind and physiology.
I worked through an entire volume of the Well-Tempered Clavier in this manner, writing in every fingering and practicing all 24 Preludes and Fugues slowly before memorizing individual fugues. Now, whenever I work on any given fugue, it's already plenty familiar to me and much easier to memorize.
It's important to point out that the number of pieces you can practice at any given time is very much determined by your practice time. If you're able to practice all day, you can indeed work your way through entire volumes and still be able to focus on individual pieces to learn thoroughly.
Most amateur pianists simply don't have much time available, so the solution is to practice just enough pieces to keep your mind active, and spend most of your practice time really focusing on individual pieces.
The important distinction to keep in mind is that there is absolutely no difference in practice between laying the foundation and memorizing a piece for performance. The only difference is that polishing a piece requires more time.