The way Facebook deduces your number is by examining those people who have stored you as a contact in their phone and who have given Facebook access to their contacts. If 50 Facebook members have a guy called "John Smith"/"Jon Smith"/"John S"/"Big J"/"JJ"/"John mob"/"John tinder" in their phone all with the same number, it doesn't take too much to figure out what the name of this person is.

Once Facebook has the number and probable name tied together, together with a graph of acquaintances, it already has a good picture of this person whether or not they have an account.

If you do have a Facebook account but you didn't link the mobile number, the graphs of your Facebook friends's contact lists will reveal what your number is quite quickly.

I assume that WhatsApp also contributes the mobile number information to Facebook's graph.

If you have multiple numbers, and really who doesn't, I don't imagine it adds much confusion to the above process.