While everybody else's answer may be correct in the context of reality, you're missing the point of the scene, if not the entire movie.
He works at the same company as everybody else, has the same position, shares the same lifestyle, the same interests and hobbies, has the same clothes, the same routines and hangouts, they share the same women, have similarly expensive apartments...
His entire "mask of sanity" is based on possessing all of the same things his peers have.
Why wouldn't he have the same number as everybody else? Bateman doesn't actually have an identity of his own. His existence revolves around emulating his peers to fanatical ends. The phone number is just another thing they all have in common, a serial number that fails to differentiate any of these characters from one another.