The A-23 virtual piano does not limit the number of keys you can play simultaneously. However, the hardware design of most computer keyboards generally restricts the amount of keys that can be read at the same time. This is what we know as Keyboard Ghosting, and it will depend on the particular key combination we are trying to press and the quality of our keyboard.
In order to increase the number of simultaneous keys our keyboard can detect we recommend getting a certified "anti-ghosting" gaming keyboard, which should reduce this effect to a minimum. Remember this won't be an issue when playing a song using our Song Mode with auto-accompaniment as you will only be required to press one key at a time!