I believe it is based on the Latin noun for 'number' which is 'numerus'. In one of the noun's cases, dative, perhaps - Latin's a long time ago for me! - the word takes on the form 'numero'. We still see this form in 'Numero Uno' to mean 'Number One'. The German 'Nr' does the same thing...ie takes the first and last letters of their word 'Nummer'.