When a noun representing a number should usually be used in the singular form as in: one hundred, two dundred, ten thousand, twenty million etc. It shouldn't be like one hundreds, two dundreds, ten thousands, twenty millions and so on.

If used in a vague, intensifying way, hundred, thousand, million, billion (also lakh, crore etc,) can be used in the plural.
Examples:
Hundreds of men.
Thousands of rupees.
Tens of thousands of dollars.
Millions of people.
Billiions of pounds.
Lakhs of students etc.

But if a modifying agent (such as a, two, ten, seventy etc.) is used before these phrases, they should be singularised.

For example,

A/One hundred men.
Three thousand rupees. (thousands)
Ten of thousand dollars. (tens, thousands)
Thirty five million people. (millions)
Six billiion pounds. (billions)
Twelve lakh students etc. (lakhs)