When you make enough money to finally be responsible for the taxes from your Associated Content income (hurray?), you'll get an email that states you'll receive no more payments until you turn in a W-9.
In the email, it states that you can fax or mail it, but you can also print it, fill it out, scan it and attach it in an email.
You'll also notice at the bottom of the email that you must put in your account number. Community Guide Marie Anne St. Jean often tells contributors to "click to your profile page, then look at the number just before your name in the URL". This is your account number. There is a space at the end of the first section of the W-9 where the account number can go.
Why does AC require them?
I have no clue. According to knowledgeable contributor (and tax mogul) Angie Mohr, no non-IRS numbers should be going on a W-9.
I did not know this when I submitted my W-9 and after consulting Angie about it, I simply submitted my W-9 without one. I included a note in the email that I had no clue what it was for or where to find it as my first email to them asking where to find it went unanswered. I never got an email stating whether that was okay or not but I started getting payments again so apparently they accepted it.
It's unclear why AC is requiring this number but so far, using Marie Anne's advice hasn't produced any further questions on the matter.