But it's not that simple. Each Cait sith is fully autonomous and loyal to Reeve, on top of having its own personality. On top of that, in FF7 Cait Sith jumps between being a radio for Reeve to communicate to the party and speaking on his own, and there's no cue to tell which is which. The details of this are never elaborated on, beyond Reeve having a "special connection" to all the automatons he creates, which depending on your source may just be called a psychic link. How Mog, the giant moogle doll Cait rides in 7, fits into all of this is also unclear
For example, the Cait Sith that sacrifices himself to form the black materia specifically states that another Cait Sith will replace him but that they're all unique from each other and have their own memories, but outside of these scenes and his introduction, it's never brought up again and almost every important moment revolving around Cait Sith after this is implied to be Reeve.