I'm trying to think how many meows my cats have. I can think of five off the top of my head, and they change their tone depending who they're talking to (they call for my wife with a different meow than they call for me). I have just always kind of thought of them as 2 year olds with damaged language centers of their brain. I just talk to them and they meow back, and I pretend that we both understand each other.

That being said, there are a few words they've come to recognize. Like when I yell WHERE MY KITTIES AT they come running from wherever they were hiding for treats and cuddles. Very, very helpful when we had to evacuate.