There's a fixed number of item stacks the PC can carry, and there are similar, but much lower, limits for craft, animals, and companions. There is not a limit for dropped item stacks on a time, as far as I know, although there is a limit to the number of items that's allowed in the "vicinity" (something like 3 * 3 or 5 * 5 world tiles) numbering a number of thousand. Note that this number includes the stacks in built structures such as walls, fences, etc., so a fence might consist of two stacks of 1 withe each and 2 stacks of stakes (where the 2 different stacks comes from two different levels of quality), resulting in a "cost" of 4 stacks.