Two schools of thought.

Check out airliners.net, do a search on MD-11 cockpits. There's one picture there taken by Sam Chui of a Varig which shows the Left, Right, and Aft Observers seat (which rotates and elevates.

Some of the MD-11 customers had a fourth seat installed, just to the left/aft of the Captain's chair.

There was also a fold-down jumpseat, just as you enter the cockpit, on the left.

So, two driving, up to three along for the ride.