Actually, this has never been stated, the only thing Faust ever mentioned about it was that Cadance was originally intended to be a unicorn. This could mean that there were only ever supposed to be 2 or it could mean that the whole alicorn ascension thing was only meant for twilight later to make it more special or rare.

Given the nature of television shows it's entirely possible that Mrs. Faust never planned that far ahead or even had multiple conflicting ideas, people need to remember that T.V. is designed by committee and writers rarely ever plan things out to a concrete detailed end when they are just starting.