Jelly is a liquid, just like water, as it fills its container. You would say "I drank much water" rather than "I drank much waters," so I'd say it's the same situation for jelly; because you aren't talking about separate masses, and you're talking about a random section from a singular mass, it would simply be jelly. Again, this parallels the water comparison. You might say "The waters are very different" when talking about two watery masses or two differently flavored waters, and you would say "I liked the jellies" if you were talking about multiple flavors of jelly.
