Both “Hairs” and “Hair” can be correct depending on the context. We use “hairs” as a plural countable noun when we are talking about single strands of hair. We use “hair” for singular countable nouns and when we are talking about hair in an uncountable context.
We usually use “hair” in an uncountable context because there are usually a lot of hairs and it is impossible to count them all.
We use “hair” when there is one singular countable hair”
There is a hair in my soup.
I can see a grey hair on my ear.
We also use “hair” when we talk about hair in general.
Oh, I love your hair! What have you done with it?
There is a lot of hair on my arm.
We only use “hairs” when we can count the single strands of hair.
I can see three hairs on my finger.
Note that when it becomes impossible to coun, then we switch to the uncountable “hair” form.
There is so much hair on my finger.