Comment 4 for bug 61514

It's not that simple; many languages have more forms than just singular and plural. For example, Russian and some similar languages have one form for numbers ending in 1 but not ending in 11, a second form for numbers ending in 2-4 but not in 12-14, and a third form for everything else. Lithuanian has a special case for numbers ending in 12-19. Czech and Slovak have one plural that's used for 2-4, and another for everything else - and so on. See "Plural forms" in the gettext documentation for more details.

debconf has never implemented this, so where possible I prefer to try to avoid relying on substituting numbers into debconf messages in a way that requires plural nouns.