God is One.
We know the number one in mathematics ("counting”).
The Divine "One" is not like the mathematical one.
In mathematics, the concept of "one" allows for the possibility of an additional one.
By the same token, "one" can be divided into parts, into fractions.
But when we say that the Divine is One, we are speaking in altogether different terms.
There can be no additional "one" to God, for He is unique–"there is no other than He" (Deuteronomy 4:35).
Not only does His Oneness deny the existence of any similar being, but in the deepest sense it denies that anything exists but that Oneness.
Finally, unlike the mathematical one, which can be divided, God's Oneness is indivisible and radically simple.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose, Chapter 13, "Patach Eliyahu-Elijah Began," by Rabbi Adin Steinsaltz