I think they're limited to 125 items, but also have a weight limit, just like your characters have. (Found that out putting piles of ingots on a vendor instead of commodity deeds. Wanted to sell piles of ingots instead of commodity deeds because EJ accounts can't use commodity deeds. Had to make the ingot piles smaller than I'd originally planned.)
The only way I know of in the Classic Client to get a count of the items held by a vendor is to change the "Item Properties Display Mode" under the Interface tab under Options on your character's paper doll to "Single Click." Then open the paper doll of the vendor and single click on its backpack. That will show the item count and weight for the vendor's backpack.