I would use this in the singular. This is because each portal will concern one supplier and it sounds more personal, because it is a fixed expression, and also because it seems to flow better in "internet speak".

"SUPPLIER PORTAL
A supplier portal is the front end of the e-invoicing or e-procurement platform which enrolled suppliers connect to via the internet.
Here, suppliers can accept purchase orders, change profile information such as bank details and addresses, flip purchase orders (see PO flipping), and raise invoices.
http://www.sharedserviceslink.com/page/glossary#supplierport...