Learn English – Why do we say “I am in a good mood” and not just “I am in good mood”

This phenomenon - the uninflected plural of numerals - is a remnant of Old English. In Old English, words like hund "hundred" and þusend "thousand" were indeclinable. That is, they did not change form based on case or number.

Today this remains but only in some limited situations - specifically, when the number word is preceded by a determiner (a, one, another number). This is also the case in English's sister language Frisian (you knew English had a sister language, right?), where tûzen, etc., don't take a plural form.

Many nouns which are normally uncountable are potentially countable in certain contexts. It’s a matter not so much of grammar as of the nature of the object to which the noun refers. There can clearly be different kinds of foil, and that makes it possible to speak of foils. Matter is a concept that that lends itself less well to pluralisation, but the Corpus of Contemporary American English has this record:

A 1992 study estimated that CSOs release between four and fourteen billion pounds of solids and organic matters on a yearly basis nationwide.

Organic and inorganic matters might in some contexts be an unwise formulation, because it could be taken to mean ‘organic and inorganic topics’.

David Crystal comments on the pluralisation of uncountable nouns in the latest post on his blog.