I believe there should be one of them for each door - it could be that one has been replaced before and it's now fallen out too...
The culprit is usually in a metal runner at the front edge of the door. The rubber fits in a way that (from memory - it's been awhile!) the curved edges fold together and basically touch - the back edge then slots into the metal running - and then when the window comes down, it slips in between the folded over edges and that keeps the window steady from rattling.
When I put mine back in, I ran a smear of silicone grease down the edges that create the slot for the glass to run in - helps prevent it from binding/grabbing on the rubber when it moves.
I also seem to remember that the 2 sides that fold are unequal - one side is bigger than the other and folds over further. It does then matter which way the rubber goes into the metal channel to make sure the glass runs in it correctly.
It actually sounds more complicated (especially in my explanation) than what it is - but it's one of those things that is hard to explain, but makes sense when you get it fitted!