Reseller Number
A Reseller Number is a unique identifier for each Apple Authorised Reseller or authorised mobile network provider who participates in Apple Business Manager. When you add a participating Apple Authorised Reseller’s or mobile network provider’s Reseller Number to your account profile (and you give that reseller your Organisation ID), you authorise that reseller to submit devices you purchased through them to Apple so their serial numbers appear in Apple Business Manager.
If you do not know the Reseller Number of your participating Apple Authorised Reseller or your authorised mobile network provider, contact them for assistance.