Weight load for a bench using 2 x 12
I ran this through the sagulator, Assuming a shelf 36", 2x12 on side, supported at the ends (floating) with a center load of 1000 lbs, and got a total deflection of .07 inches.

1200 lbs still read as acceptable,  1250 went borderline.

So for supporting one or two humans within normal weight parameters, you should be good.  I'd be more interested in the legs and how they are connected.