any one / anyone
A single person or thing from a group versus the indefinite pronoun.
Write anyone as one word for “anybody”: Anyone can apply. Write any one as two words when one is a number you could point to: any one of the keys, any one candidate. If you can add single without wrecking the sense, you want two words.
Examples
Use Anyone who arrives late will wait.
Use Any one of these three proofs will do.
Avoid Any one who arrives late will wait.