From Laura K. Lawless,
Your Guide to French Language.
When you want to make an estimate or take a guess, you can use approximate numbers. Most approximate French numbers are formed with the cardinal number, minus the final e (if any), plus the suffix -aine.
une dizaine about ten (note that x in dix changes to z)
une douzaine a dozen
une quinzaine about fifteen
une vingtaine about twenty
une trentaine about thirty
une quarantaine about forty
une cinquantaine about fifty
une soixantaine about sixty
une centaine about a hundred
un millier about a thousand
Approximate numbers are treated grammatically as expressions of quantity, and like all expressions of quantity, approximate numbers must be joined to the noun they modify with de.
une dizaine d'étudiants about 10 students
une quarantaine de livres about 40 books
des centaines de voitures hundreds of cars
des milliers de documents thousands of documents