“Hundred”, “thousand”, “million”, etc., when they are used in the names of numbers (i.e. when they are preceded with a number) are always in the singular and usually not followed by “of”, for example
A number is generally followed by “of” when we enumerate something else than a noun, for example:
The only case when “hundred”, “thousand”, etc. take the plural form is when they are used in the sense of “an unspecified number of hundreds/thousands/…”, e.g.
with themselves on a rainy Sunday afternoon.” —Susan Ertz
The case of a few, several, many…
What do we do when we want to say “several hundred(s)”, “many thousand(s)”, “a few million(s)”, and the like? The number is unspecified, so it seems we should use the plural form, but still, it is a number, which indicates the singular form.
Indeed, some speakers do say “several hundreds of”, “a few millions of”, etc. However, the variant without “-s” and “of” is much more common and considered acceptable by all speakers, whereas the other variant is usually considered unnatural by those who don’t use it. It is therefore advisable to stick to the singular form: