8, with 4 under each seat (4 x 11 = 44 lbs), to keep you under the cargo limit of 50 lbs/seat.
I didn't find an issue with the HIGE/HOGE for the temps and altitudes given, but the fuel load would be right at it's limit for a Beta w/o the aux tank (with the aux tank you'd load as much as you could w/o going out of CG on the outbound portion of the flight).
Change the question to "Should you take an R22 Beta with no aux tank to pick up a bunch of gold bars at ~8000 PA..." If you didn't have the aux tank, you'd be cutting very close to the required 20 minute reserve.
No aux tank:
depart with full fuel (19.2 gal, 1136 lbs), arrive with 11 gals fuel (1088 lbs), add 88 lbs (1175 lbs), return with 3 gal fuel (~20 mins).