See thread title.
Latin numero, ablative of numerus, “number.”
Are you sure it isn’t from the French/Old English “nombre”.
Just to throw a third option into the mix (but this time with a cite 
) my dictionary says it’s from the French numéro.
Which is presumably from the Latin anyway.
The OED says it’s “post-classical Latin”. It was also originally written with a superscript “o”: N[sup]o[/sup] or n[sup]o[/sup].
Nice people (like me 
) also underline the superscript O - although it looks horrible on a computer screen.