Just a few concepts i've seen thrown around:
A concept; take your # of SKU's multiply by total inventory size. divide that number by four. Example: 3 million dollar inventory, 4 thousand skus (3x4=12) divided by 4 (12/4=3). So you'd need 3 SKUs.
Another concept: Do you buy Capital, Indirect, and Direct materials? A buyer for each category as to allow specialization.
Another concept; Total number of Purchase orders (take for example, 4000) divided by 1,000 PO's per person (averages 4ish purchase orders a day). So, for 4,000 POs, you'd have 4 buyers.
Really, your biggest challenge won't be finding the criteria; you're biggest criteria will be justifying a *supposed* non value-added position to an organization that is probably hell-bent at reducing labor costs.