Пианино, пальто, пенсне and some other words which were derived from other languages are neutral and indeclinable. Одно пианино, два пианино, одно пальто, два пальто. So it's always пианино.
Thanks! In school, I only learned рояль, but most people, including myself own a пианино. I think it's funny how my boss always makes the distinction in English. I think of either kind as just a piano.