In the Old Testament, the Shema was a main priestly blessing that is pronounced by Jews even today. Deuteronomy 6:4 declares, "Hear O Israel, our Lord God is one Lord, and thou shalt love the Lord thy God with all thy heart." The word "one" can be understood in several ways. For the most part, people think of it in terms of monotheism—there is only one God numerically—but the ancient text was thinking in terms of there being only one true and faithful God. Almost all ancient people believed or supposed that there were many other gods, but each nation felt that their God was preeminent. So the primacy of the one Lord may have much less to do with monotheism than the supremacy of their God, Jehovah, over all others. One was not allowed to bow down and worship other gods. Theirs was the one and only God that they loved and served with all their heart, might, minds, and strength.
In the Book of Mormon, the issue of monotheism comes up here in Mosiah 15, and again in Alma 11 when Zeezrom and others challenged the Nephite teaching that Jesus and God the Father are two beings, with there being just one Eternal God (Alma 11:44). While the careful wordings of these sacred statements about the awesome nature of deity are in some ways intentionally mysterious, a lot of how these statements should be understood comes back to the question of how the Shema in Deuteronomy should be understood. It has more to do with exclusive loyalty than numerical counting.