The problem is that, according to RAW, you have either multiple or one US depending on your reading. I read it as one, but a lot of people read it as one for each limb. You obviously read it as one, and Robby reads it as one/limb. Even though that's wrong I don't care Ejo you're wrong. That would be more relevant if he was in this thread....
The Monk doesn't change what USs are, just what they do. They are always a combination, RAW.
These things led me to my US houserule set, which attempts to fix the major problems.
First off, IUS now gives you proficiency, and a scaling damage like the Monk.
Second off, USs are manufactured weapons if you take IUS, and natural if you don't. Period. No combination status. You have one US per body. The mechanics are as per weapons, so iteratives, TWF/MWF, whatever.