I don't think there is a limit on how much one can improve (or not improve) during any frame of time. Even in a moment everything can change with the right idea. To date, the most measurable skills I have acquired are in gaining clear, reusable concepts about playing that can be relied fully upon (and worked with for building upon) by me at any moment. A person can play pieces, exercises, spend hours at the instrument, etc., but if reliable -or at least workable- concepts are not being carved out, all that time is not amounting to as much as it could be.From what I am reading, 10-20 pieces is your chosen "method of improving" in a set amount of time. I am interested, where does this number of pieces and the idea of using pieces come from?