From personnal experience, I have always seen people using “N°” or “n°” (both using DEGREE SIGN U+00B0) written by hand, on computer or printed in books/newspapers.

Selon le « Lexique des règles typographiques en usage à l’imprimerie nationale » qui fait référence, on trouve dans la liste des abréviations :

soit :

• n° si on n’a qu’un numéro
• n°^s si on liste plusieurs numéros. (Reste à savoir comment obtenir le « ^s ».

Using the degree sign is often done because it is directly available on a French (AZERTY) keyboard. That gives n° and N°. It looks like the famous perfume named N° 5 uses a degree sign:

Doing it is however sometimes considered incorrect because the degree sign is technically a small zero while the upper o is the vowel ending the word numéro. Using the degree sign also makes the plural unaesthetic to say the least: n°ˢ or n°^s.

To get a consistent display, you then need to use a superscript “o”, possibly followed by a superscript “s” if plural:

Numéro: HTML: n^o or N^o, UTF-8: nᵒ or Nᵒ

Numéros: HTML: n^os or N^os and UTF-8: nᵒˢ or Nᵒˢ.

Sometimes, the masculine ordinal glyph is used as in nº and Nº. It is not recommended because depending on the font used, the o might be underlined which is not a French thing, and there is no matching “s”.

Similarly, the “Numero glyph” (№) is unused in French because the “o” is underlined and there are neither lowercase nor plural forms.

Capitalization follows the generic rule, if numéro is starting a sentence, the initial N should be in uppercase, otherwise, a lowercase n is used.

Note also that a space is required after n^o, e.g.:

N^o 4