Does Rambam teach one should not say "God is one" but rather "God is not multiplicity?"
https://en.wikipedia.org/wiki/Maimonides See segment labeled "Negative Theology."
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most likely means like the shaar yichud states
When we describe Him as One, we mean only the negation of any plurality. But the true Unity, cannot be described by any attribute that would connote in His glorious essence any plurality, change, or variation... But the denial of such properties to Him is undoubtedly true and appropriate to Him. For He is above all attributes and forms, similarity or comparison. Therefore, you must understand from these attributes that they refer to the negation of their opposites.
(Marpe Lenefesh commentary: It is better to negate ascribing to the Creator attributes which are lackings on Him. For example, it is more correct to say that the Creator is not plural, not non-existent, not created, which are opposites, and which are more true than saying and affirming on Him that He is the "true Unity", "permanently existing", "eternal", because we are not capable of understanding what is true Unity,...)
i.e. believe God is one. But keep in mind that you don't know what it truly means so as not to have the wrong conception.