Bonjour Shelli,

For number like "21, 31, 41, 51, 61 et 71", you will use "...-et-un" (1990 Spelling Reform) or "... et un".

For numbers from "101, 201, 1001 etc", you will use "...-un" (1990 Spelling Reform) or "... un"

Here's a link you might find useful (if in any doubt): http://www.chiffre-en-lettre.fr/

I hope this is helpful.

Bonne journée !