Use hundred when there is a specific number, and use hundreds when you don’t
know or can’t count how many. This rule also applies to thousand/thousands and
million/millions.
▶️ This skeleton is thousands of years old.
▶️ This skeleton is three thousand years old.
When saying numbers, always use the singular form:
▶️ 250 = Two hundred and fifty
Two hundreds and fifty
▶️ 7,812 = Seven thousand, eight hundred and twelve
Seven thousands, eight hundreds and twelve
▶️ $10,000,000 = Ten million dollars
Ten millions of dollars