I looked at the Type Certificate Data Sheet for guidance:
http://www1.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/0f6b1b49edba583b862573ca00695c18/$FILE/A-759.pdf
and it clearly is only certificated for 2 seats - so I would think the answer is 2 seats = 2 passengers.
Eli