None. Go nuts.
However, the weight of the item limits the number you can stack on a character.
Note that some items do not stack. Example: Gear pieces, Ancient Relic Shard, some loot boxes, etc.
However, some items that don’t usually stack, will stack when placed in the warehouse. Ancient Relic Shard, Gear pieces, do stack in the warehouse. But the warehouse has a weight limit which restricts how many items you can place in it, although the slots are huge. And it is readily accessible from major cities or Transaction maids.