How much Load Capacity does a 12' 2x12 board have?
Putting those numbers into the sagulator, a 12' 2x12" Ponderosa pine board lying flat could support 100-125 lbs with about 0.2" of sag in the middle.

If it were on edge, it could support several thousand pounds. The sagulator says borderline sag began at about 12,500 pounds, but that was beyond their stated target sag of 0.02" / ft. I found that target sag at about 7000 lbs.