This is the same as your other question about here's vs here are, and wasn't there another one about there's vs there are?
Using the singular 'is' instead of the plural 'are' when referring to something countable in the plural is grammatically incorrect, but you will hear it a lot; although not from me.
Imagine I am walking along with a friend. We see two people coming towards us in the distance. I wave to them and they wave back. My friend says "Who's that?" I reply "It's my parents". That is a perfectly natural exchange even though there are two people but both the question and the answer appear to use the singular (Who's = Who is, It's = It is).
I wouldn't use that in reply to "Who's that?" I might use it in reply to "Who are they?" but that wouldn't be the natural question (BrE) in my scenario.