Hi Sundi,
I found a very helpful explanation
online that I have posted below. This site seems to be very helpful also, I've bookmarked it for future use! Thanks for posting this question...I've learned something new!
---------------------
Grammar Trap: Anyone vs. Any One
How do you decide when to use which word? It's not that tricky.
Use "anyone" when you're making an indefinite reference and mean "any person at all."
Example: It's so simple that anyone can figure it out.
Use "any one" when you are referring to a specific person or element in a group.
Example: It's so simple that any one of you can do it.
Tip: The same "one word if it's indefinite and two if it's specific" rule also works for "someone" vs. "some one," "everyone" vs. "every one," "sometime" vs. "some time," etc.
--------------------------
Hope it's helpful to you.