While Mozart did not compose a set of '20 Piano Compositions' as a single opus, he wrote numerous individual pieces that, if collected, would represent a vast and diverse output across his career. These would encompass sonatas, variations, rondos, and smaller character pieces, showcasing his unparalleled mastery of melody, harmony, and form.
Developing refined articulation, expressive phrasing, and a clear melodic line are paramount.
20 Piano Compositions is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
20 Piano Compositions is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.