There is a simple choice to make.
Is God singular? Or is he plural?
When a person states that “elohim” has a plural ending, the question remains: is God singular, or is he plural?
When a person states that there are [rare] plural words for God or that plural pronouns are [rarely] used, the question remains, is God singular or is he plural?
There is no need for the obfuscation of those who speak of compound or complex unities, or trinities or families. The choice is amazingly more simple than any of that. Is God singular or plural?
If there is one being called God the Father and another called God the Son, then there are two Gods. If there is an addition of God the Holy Spirit, then there are three Gods. If the plural usages of Hebrew words and pronouns are understood in a narrow way, then God is plural and it is fine to say “Gods”. With that logic, the singular Hebrew word is eloah and the plural is elohim in the same say that the singular word is “chair” and the plural is “chairs.” If the word “one” is used with such an understanding, then it only means “one” in the sense of a group. So at the core, we are still talking about something that is plural. Even if they were to say, “actually we do believe in one God,” the term “one” must be understood as a group, and the term God must be understood like the word “sheep” – there’s no such word as “sheeps” when talking about more than one of them. We can talk about “one God” and it would be equal to a herd or group of “God” (although that is a mutilation of the English language … for now anyway). It’s plural.
If God is singular, then the above understanding is polytheistic and idolatry (and also an abuse of the Hebrew language).
Those that believe in God the Father and God the Son (whether or not they include God the Holy Spirit) are not monotheists, those who acknowledge a singular God.
As a funny aside, even those that only believe that God is singular, but that there is also “a god of this world/age,” being the devil (2 Corinthians 4:4), are not monotheists either. They may only worship one of the deities, but they still believe there are two.