Quantity of repertoire does matter. If a pianist has played ten or fifteen Beethoven sonatas, most of the time he will have a better understanding of the style and will have an easier time approaching a work with similar elements than a pianist who has only played one or two. Quality is important, but when one is studying it is very important to be familiar with as much repertoire as possible, not only by listening to it but by actually working on it for a few days at least. It is a good idea to start with much more repertoire than one will play in the school exams or recitals and eventually pick a few of the pieces and bring them up to performance level. As long as it doesn't interfere with practicing your normal repertoire it is a good idea to read through staples of the piano repertoire, and try to work out how the difficult passages would be played. It is also a good idea for when one is burned out from over-practicing the same repertoire all the time.