In Genesis 2:24 we are told that a man shall leave his father and his mother and be joined to his wife and they shall become one (echad) flesh.
In Genesis 11:6 as G-d describes the post-Flood environment where the people were attempting to build the tower of Babel He says
The LORD said, "Behold, they are one people, and they all have the same language. And this is what they began to do, and now nothing which they purpose to do will be impossible for them.
We see from these and other passages that echad can mean a unity of multiple items: one flesh, one people. It does not necessarily mean a singularity: one and only one.
There is another Hebrew word for a singularity: yachid. This is Strong's # 3173 and it comes from the root word yachad which means "to be united". The first three times it is used is in reference to the binding of Yitz'chak, the akedah, in Genesis 22:2, 22:12, and 22:16 where-in Yitz'chak is described as Avraham's "only" son.
The shema does not use yachid to describe G-d it uses echad but leaves us with the question: one what?
It is the journey of each of our lives to discover some portion of the infinite nature of G-d's unity by studying His Word (2 Tim 3:16), His creation (Psalm 19), and the work of the Holy Spirit (1 Peter 1:2) within the body of believers.
To Him be the glory forever.