Piano overtook the Harp chord by the time Mozart had arrived on the musical scene in Vienna. I think he was the first accomplished composer to write music for piano. By the time of Beethoven, the piano had technically developed so much, that it was clear that it couldn't just be an instrument to replace its predecessor, the Harp chord. It became a fashion to write concertos, symphonies, sonatas with piano as a lead instrument rather than a violin.
Later in the century, when it was eccentric Frederic Chopin's time to glory, piano was recognized as a stand alone solo instrument, thanks to efforts by Schubert, Schumann and Liszt. Piano, as opposed to violin or guitar offers a clearer sound without overtones. Also, compared to their string counterparts, piano in theory can play 10 notes at a time, while violin (1-2) and guitar (6) are far less rich. The only major disadvantage of piano sound is absence of clear micro-tones.
Since piano was recognized as a stand alone instrument, the number of pianos in a concert is strictly less than or equal to one. To my astonishment, I heard a waltz by Chopin, which can be arranged for a string quartet, a full orchestra (I already had this version), a single piano and interestingly, for 2 pianos. After listening to it for a number of times, I figured out that both the pianos are playing a similar role in terms of importance which was further shocking. Here is a link.
And finally, OT but I created a website for my group.