The account number can be any number. It does not have to be an actual bank account number. The IRS uses this field to distinguish between returns when corrections are filed; especially if more than one of the same form was filed by the same payer for the same recipient. The account number is an IRS requirement for electronically filed 1099 forms.
For example, ABC Company issued two 1099-MISC forms for John Doe. Each forms needs a unique account number. One form could have 'ABC123-01' and the other could contain 'ABC123-02'.