Begin with a simple string quartet (Mozart, Haydn, or even C.P.E. Bach). That gives you four lines in three clefs. Then move up to some J.S. Bach Art of the Fugue. These may be up to five lines and they are more dense than those quartets. Then, at the same time, try some early symphonies of Mozart that have, for example, strings, two horns, and one woodwind. They would have about eight lines. After this, try music for larger chamber ensembles like the Mendelssohn Octet or Beethoven Septet.

In playing the string quartets or Art of the Fugue, your goal is to play all of the notes. As the scores get larger, you will encounter doublings and also passages that, by their nature, are impossible to totally realize on the piano. In those instances, find the dominant parts and harmonies and play those, remembering that, in performance, some sounds come out more than others. The task, as you know, is sight reading and playing, but then it is also judgment and thinking on your feet.

There are books on score reading. If you can get to a good music library at a university or conservatory or to a good music store like Patelson's in New York, you should find some helpful books.