This rubber seal is required only if you do not have windows. The seal hides the pinch weld on the inside of the van. Same seal works on both sides, left or right.
The seal is required to set the gap of the glass to the body of the van.
If you already have fixed windows on your Sprinter, you can re-use the seals you have.
Pricing is per seal, add 2 if you're buying a pair of slider windows. ( Item #2 in the pic)