Notes from ¿Qué tal? - Los números 100 y más

Continúe la secuencia: noventa y nueve, cien, ciento uno, …; mil, dos mil, …; un millón, dos millones, …
100 – cien, ciento
101 – ciento uno/una
200 – doscientos/as
300 – trescientos/as
400 – cuatrocientos/as
500 – quinient os/as
600 – seiscientos/as
700 – setecientos/as
800 – ochocientos/as
900 – noveciento s/as
1.000 – mil
2.000 – dos mil
1.000.000 – un millón
2.000.000 – dos millones

Ciento is used in combination with numbers from 1 to 99 to express the numbers 101 through 199: ciento uno, ciento dos, ciento setenta y nueve, and so on, Cien is used in counting and before numbers greater than 100: cien mil, cien millones.
When the numbers 200 through 900 modify a noun, they must agree in gender; cuatrocientas niñas, doscientas dos casas.
Mil means one thousand or a thousand. It does not have a plural form in counting, but millón does. When used with a noun, millón (dos millones, and so on) must be followed by de.