Numerals like dozen, hundred, thousand, million and such like are sometimes used in singular, sometimes in plural. So, we have hundred vs hundreds, thousand vs thousands, and so on.
So, when do we use them in singular and when do we use them in plural?
We use them in singular when they make part of a concrete number:
two dozen eggs = 24 eggs
five hundred people = 500 people
twenty thousand birds = 20,000 birds
ten million dollars = $10,000,000
We also use them in singular if the number isn’t very precise:
several hundred people
We use them in plural if they just mean ‘a lot’:
dozens and dozens of people = very many people
they arrived first by/in the dozens, then by/in the hundreds = they arrived in large groups
thousands of cows = more than 2000 cows
thousands of times = many times
he could have saved millions of lives = over 2,000,000 lives
it’s worth billions = more than 2,000,000,000 dollars/euros/etc.
hundreds of billions of stars = more than 200,000,000,000 stars
Here’s the video version: