Maimonides' Second Principle is the following:
We believe that this Primal Cause [G-d] is One. [His is] not like the oneness of a pair, nor like the oneness of a species, nor like man, whose complex oneness may be divided into many units, nor like the oneness of a simple body, which is one in number but may be divided and separated without end. Rather, He is One with a Oneness that knows no parallel in any manner. This is the Second Principle, as affirmed by the verse (Deut. 6:4): "Hear O Israel, G-d is our Lord, G-d is One."
Just so we have the Hebrew text for Deuteronomy 6:4 so there are no doubts:
שְׁמַע יִשְׂרָאֵל יְהוָה אֱלֹהֵינוּ יְהוָה אֶחָד
Although I have had some Christians explain that the word אחד means "unique" because it describes Jesus' unique trait of being triune, that is simply not true because Hebrew has another word for "unique": יחיד. Since that doesn't work too well, Christians then will cite two examples of אחד accompanying a compound unity:
וַיִּקְרָא אֱלֹהִים לָאוֹר יוֹם, וְלַחֹשֶׁךְ קָרָא לָיְלָה; וַיְהִי-עֶרֶב וַיְהִי-בֹקֶר, יוֹם אֶחָד.
And God called the light day, and the darkness He called night. And there was evening and there was morning, one day. -Genesis 1:5
וַיִּכְרְתוּ מִשָּׁם זְמוֹרָה וְאֶשְׁכּוֹל עֲנָבִים אֶחָד....
"....and cut down from thence a branch with one cluster of grapes." -Numbers 13:23
If you took these two examples alone, you would think that the word אחד always refers to a compound unity. Upon closer examination, there's another story to tell. Let's first keep in mind that out of the other 960 usages of the word אחד in Hebrew Scriptures, which constitute as over 99% of the word's usage, אחד is unambiguously used as a numeric adjective referring to the number "one."
We should take a look at these two "anomalies" and see if we can get past the confusion. Let's take an example in English, simply because the concept carries over in Hebrew. When you're talking about a dozen eggs, are you referring to the twelve eggs or the specific grouping thereof? Although the word "dozen" implies that there are twelve eggs, you are still referring to the single entity: a dozen. It is the same principle within these verses. אחד is modifying the word אשכול (cluster), not ענבים (grapes). Even in these two instances, you are referring to the grouped entity, not the components within the entity. No matter how you look at it, one is still one.
So let's take another look at the phrase יהוה אחד in Deuteronomy 6:4, which means "G-d is one." In the present tense in Hebrew, there is no conjugation for the verb "to be." The meaning of "to be" in the present tense is always implied by context: the absence of a verb. Since we know that this verse is in the present tense, the word אחד is a de facto stative verb, meaning that אחד in this passage unambiguously translates to "is one." Although you had the word ענבים in Numbers 13:23 in order to attempt to create some doubt, there is nothing in this declaration to suggest a compound entity. Finally, anytime the word יְהוָה, the personal name to denote G-d, is used approximately 6,800 in Hebrew Scriptures, there is never, ever a plural verb or adjective used to describe G-d; it is always in the singular.
G-d is not a triune deity. As the grammatical rules of Hebrew dictate, יהוה אחד unquestionably means "G-d is one." Any Christian who purports otherwise does not believe in monotheistic notion of G-d that is clearly laid out in Hebrew Scriptures.