Piano Forum

Are you asking about playing polyphonic music or composing polyphonic music, or both?

J.S Bach is definitely good to study for both, especially for playing polyphonic music with his 2 part and 3 part inventions, which allows one to try pieces in different number of voices from 2 to 3 (then 4 with the WTC).

Frescobaldi's Fiori Musicali (end of Renaissance beginning of Baroque) is a great resource as well, highly influential on most baroque keyboard composers, Bach studied this work for sure. If you're up to the challenge you could try transcribing some a capella motets or madrigals from the renaissance, I recommend Thomas Tallis since a lot of times he combines both polyphonic and homophonic textures within the same multi-voice works.

Sergei Taneyev (Late Romantic) is a great resource as well, maybe not so much for playing on piano (he has a very difficult prelude and fugue for piano, opus 29), but he wrote a book about invertible counterpoint that's kind of hard to read, but he has a chapter summarizing the rules of strict counterpoint which I recommend checking out.

All the great composers will have polyphonic or contrapuntal textures
polyphony always has counterpoint, but counterpoint doesn't always mean polyphony: polyphony is when the foreground, or the dominant line, is moving around from voice to voice, which requires counterpoint. But a homophonic piece could have a lot of independent inner textures (counterpoint) that never rise to the foreground (a lot of Chopin and Rachmaninoff)