The word for 'one' (одќн) is a numeral that is declined as a special adjective. In some contexts, it has an adjectival meaning as well: 'alone, by oneself'.
When used with вІ referring to more than one person, it has the plural form однќ, but when used with вІ referring to one person it may have either the Plural form однќ (which is what you would expect for the grammatically plural word вІ) or the Singular forms одќн (speaking to a man) or однђ (speaking to a woman).
Онќ живџт однќ. They live alone.
Онђ сидљла тђм однђ. She was sitting there alone.
ВІ однќ? (одќн? однђ?) Are you alone?
Ћн одќн знђет. He alone knows.