How do you say '120' in Spanish?

Mil ciento diez or mil cien diez if you're just saying it as the number 1,110 and not 1,110 of something. (Explanation below.)

Mil means 1,000.

ciento means 100. When you're saying 100 alone as the number 100 and not 100 of something, you can say "Cien".

Diez means 10.