Adjectives:
Adjectives are invariable.
Correct: I have twenty thousand dollars.
Incorrect: I have twenty thousands dollars.

Compare
I have ten dollars.
I have a ten dollar phone.
Notice the dollars in the first sentence is plural and singular in the second. This is because the first is a noun and the second is an adjective.

Structure:
Adjective (invariable) + adjective (invariable) + noun (variable)
Be careful with dozens, hundreds, thousands and millions.
They can also be nouns so they are variable.
So: I have thousands.

Sample
I eat big Italian pizzas.