Humanoids with more than 2 pairs of arms weren't an unknown concept in human culture. From ancient mythology, there are the hundred-armed Hekatoncheires (who have their own thread here) and six-armed Gegenees from Greek myth, the many Hindu deities that were depicted with as many as a thousand arms (such as Avalokiteśvara).
And from modern times, there's the six-armed Spiral from the X-Men comics...
... the few times Spider-Man) ended up growing four extra arms (as one step among many in a gradual mutation into a "Man-Spider"...
... and the Shokan race from Mortal Kombat.
Science considers any real-life instances of this as falling under the umbrella of a medical disorder named "polymelia", which applies to all limbs rather than just arms. Personally, I'll draw upon both that term and the name of the aforementioned Hekatoncheires to coin the terms "polycheires" and "polycheirid" (lit. "many-handed one") for all more-than-two-arms humanoids, with "poly-" being replaced with the appropiate numerical prefix when a particular number of arms is specified (e.g. "tetracheirid" for four arms, "hexacheirid" for six arms, and so on so forth).
Now, with that introduction done, we come to the big question: How does one design an anatomically correct musculoskeletal system for a polycheirid? What I've managed to gather on the subject makes it seem as if even adding a single extra pair of arms would require significant modification to the human(oid) torso's muscles and bones to be possible. Most importantly, the idea of having two or more arms sprout from the same spot is apparently impossible from an anatomical standpoint, despite it being a somewhat common design approach in modern fiction.
Supposing we do figure out the anatomy, what can one expect from the polycheirid in terms of the arms' movement range, ability to apply force, and so on so forth?
PS: For the record, I would've added this to the Anatomically Correct Series, but the page (and the entire Meta Worldbuilding section) seem unaccessible to me at the moment.