They are the one/ones

"They are the one/ones who helped me."

Which one is correct? I have seen "one" is commonly used rather than "ones".

"They are the one who helped me" doesn't make sense. We use "they" when referring to one person if we don't know that person's sex. However, if this is the person who helped you, then you know if it was a man or a woman.

He was the one who helped me.
She was the one who helped me.
They were the ones who helped me. (If more than one person helped you.)