I would use ½ liter. :-)
One 2 liter bottle
It's a half of a liter
It's a two litre bottle of Dr pepper. ½ litre is not enough to properly braise the ribs. That's what makes them tender and they will absorb the 32 flavors from the DP.
To clarify my last answer, it takes a while to simmer (braise) the ribs till tender. The DP will cook off. A ½ ltr would be gone quick and ribs wouldn't be tender.