Reseller Number
A Reseller Number is a unique identifier for each Apple Authorized Reseller or authorized cellular carrier who participates in Apple Business Manager. When you add a participating Apple Authorized Reseller’s or carrier’s Reseller Number to your account profile (and you give that reseller your Organization ID), you authorize that reseller to submit devices you purchased through them to Apple so their serial numbers appear in Apple Business Manager.
If you don’t know the Reseller Number of your participating Apple Authorized Reseller or your authorized cellular carrier, contact them for assistance.