BTW I consider myself an amateur pianist cause it is not my source of income, actually it's a source of expense.
How do great pianists build such a massive repertoire? Not only can they play the pieces, they master everything they play.
For the question "how many pieces at once?" there are a number of variables, such as:Is the pianist a serious student or a casual pianist?How much practice time is available per day or week?What is the objective? Preparing for annual auditions, meeting music degree requirements, planning repertoire for a senior recital, playing for pleasure, doing recording projects, etc.?What is the pianist's level and learning capacity?Etc.So as you see, an answer might suit one pianist but not another. It's very difficult to generalize. I do believe that it's more important to ensure that there is always a mix of styles present. That is, even if one practices only two pieces at a time, then one might be Baroque while the other is Impressionistic. Of if somebody is doing 10 pieces a year, then to the Baroque and Impressionistic, one should add works of the Viennese Classical, Romantic and Modern styles as well. By playing all of the musical styles which make different demands on the pianist, one builds a more versatile technique which then enhances artistry, speed of learning, and the expansion of one's repertoire.David