It has six, 4 go up and 2 go down. That's right, two seats actually eject downward. B-52's have two decks with the top deck having 4 seats, the two pilots facing forwards and the EWO and tail-gunner seats facing aft on the upper deck (no more tail gunner, so they leave it empty or for relief crewmen). Right below the upper-deck are the two Navigators, sitting in what is basically a windowless box with their instruments and two ejection seats that go the wrong way, down. I think they need be at 1000AGL for them to survive. If you pay close enough attention in Dr Strangelove James Earl Jones is one of the Navigators and sticks his head through the hatch in the floor to listen to Slim Pickens give his rousing final speech to his crew. B-52's were not the only aircraft with downward-firing ejection seats, early models of the F-104 had them too.