Hi TimHomer,
Each customer you enter already has a unique number associated with it called the RecordID. To view this attribute, expand the Customers and Contacts grid to show recordID and EntityRecordID by clicking the gear on the bottom right and updating the grid. The recordID will correspond to a contact while the EntityRecordID corresponds to the customer.
I hope this helps! If this is not what you are looking for and want a custom field of reference numbers let me know.
Radwan