Is their singular or plural

Answer 1

There IS a singular and plural

Singular: Referring to only one thing or person.

Plural: a word in the form in which it potentially refers to something other than one person or thing; and other than two things if the language has a dual form.

I think the previous contributor misunderstood your question!

'Their' is plural. It means 'belonging to them', and 'them' is plural.

Colloquially, 'their' is often used to mean 'his or her', which is singular in meaning. But the word 'their' is still plural.

If you had used quotation marks in your question, to indicate that the word 'their' was what you were asking about, you would not have been misunderstood.

Answer 2

"their's" is not actually a word in English. I think you're looking for "theirs" as in "Mike and Kelly own that house. The house is theirs." Theirs is not singular or plural; it just lets the listener/reader know that the house is owned by more than one person (Mike AND Kelly). Possessive pronouns in English rely on the quantity of their antecedent (what they replace). You could easily say "Mike and Kelly bought some eggs. The eggs are theirs."

Now, if you meant "there's" as in "there is a house on the hill", then there is also a numberless pronoun because it can be any number. The phrase "there's" is a contraction of "there" and "is", and because "is" is singular, "there" in this case is singular, too.

You could just as easily, however, say "there're", which Firefox spell check doesn't like, but which I argue is an accurate representation of what speakers of English do, which is to make contractions everywhere! "there're" = "there are" like "there are three fish in the pond".