Nombres

Notice the et in the middle of 21, 31, 41, 51, 61, and 71.

21 = vingt-et-un
31 = trente-et-un
41 = quarante-et-un
51 = cinquante-et-un
61 = soixante-et-un
71 = soixante-et-onze
80 = quatre-vingts - notice the s at the end.

81 and 91 don't have et:

81 = quatre-vingt-un
91 = quatre-vingt-onze

In Swiss French:
70 = septante
80 = octante
90 = nonante

Notice that multiples of 100 have an ending -s, but there is no ending -s if cent is followed by another number.

144 = cent quarante-quatre

200 = deux cents
233 = deux cent trente-trois

Mille is never pluralized. Million (million) and milliard (billion) do pluralize, and they keep their ending -s even when followed by other numbers. Also, unlike cent and mille, million and milliard must be preceded by a number.

1 000 000 000 = un milliard

De must appear before nouns for million and milliard.

J'ai un million d'amis

Notes taken from https://www.duolingo.com/skill/fr/Numbers-2