@dangerousbrick already mentioned the vehicle max stack size being 40, and I think that's correct.
You can manage/manipulate these limits by making use of the PC max inventory stack capacity as well, so you place 40 (or, for more ease, slightly less) stacks of light items into your PC inventory and the heavy stacks on the vehicle.
In the case of looted clothing, you may want to spend a few days turning the smallest pieces of clothing into bandages to reduce the number of stacks (assuming you have a use for a lot of bandages, but what other use do you have for huge piles of damaged clothing).
In the extreme case you may even fight your inner pack rat and leave some of the most useless stuff behind for a future trip to the area (that's likely never to happen). I know from my own experience that this is really hard to do, though: If it ain't nailed down it has to come with me (unless I can pull out the nails, in which case those items come as well).