Final -s signals the plural form of most nouns.
Most nouns need an added final -s (sometimes -es) when they label more than one person, place, or thing.
this captain ___ these captains
Natural -s nouns have to end in -s even when they don't represent a plural quantity, so these nouns add -es when they become plural.
a mansion ___ several mansions
an insult ___ a few insults
one loss ___ three losses
But a few nouns don't need -s to represent a plural quantity. They form their plurals in some other way. These are irregular plurals.
a glass ___ several glasses
that lens ___ those lenses
a child ___ many children
one man ___ ten men
this woman ___ these women
that person ___ those people
Tip: Except for the natural -s nouns, singular nouns do not end in -s. Except for the irregular plurals, plural nouns end in -s.
A prepositional phrase beginning with of often requires a plural noun, even when the word modified by the phrase is clearly singular.
One (of my shoes) is covered with mud.
I'll wipe it with the last (of your rags).