One of the perennial objections to the Five Ways is that they don’t even show that there is one God. The objection is sometimes supported by noting that Aristotle did not think there was one first mover. The objection is superficially true, substantially false, and misplaced.
It’s superficially true in the sense that the question of the number of divine beings is not addressed in the Five ways- but to address such a question is out of place, since one cannot address the question of how many X’s there are before he has a reason to think there are X’s at all. In the case of God, the question of “the number of divine beings” takes on the added difficulty of how one can even speak of number among non-material and therefore non-quantitative beings.
The objection is substantially false, however, since the proof of what is supremely one is included in the Fourth Way. Note that the Fourth Way does not specify exactly what sensible reality it begins with but only gives examples- and then says that it extends to “other such things”. But since three of the four examples given are convertible with the one said of being, the Fourth Way proves the existence of something most of all one.
Why not just include “one” literally in the in the Fourth Way, as opposed to by including it necessary implication? The best reason is that the word “one” is too heavily weighted toward the “one” of quantity, and that to include it would likely mislead the reader.