As you point out, “country's” is the possessive form of “country,” singular. The possessive form of the plural would be “countries'…” with an apostrophe after the “s.”
In this regard, is countries plural or possessive?
Singular and Plural Possessive Nouns
Is Country singular or plural?
Basically in English, there is a group of nouns which can be either singular or plural depending on what you want to say about them. So for example, if we're looking at the United States the country we would normally use the singular verb so for example – “The United States is a very big country.”
What is the possessive form of you?
The possessive forms of you are your (used before a noun) and yours (used in place of a noun). The reflexive forms are yourself (singular) and yourselves (plural).
Is the word us plural?
In English, the plural form of words ending in -us, especially those derived from Latin, often replaces -us with -i. There are many exceptions, some because the word does not derive from Latin, and others due to custom (e.g., campus, plural campuses).