For SMEs we think it's likely to be appropriate for there to be only one organisation associated with the Supplier.
For larger organisations who work nationally, they may like to create an organisation which represents each of their independent business units or regions.
The benefit of having one organisation or one organisation per business unit, is that the Supplier's Admins can easily keep track of their obligations in one place and can produce organisation or business unit wide reports for their own compliance and CSR requirements.