An addon that just shows next to an item looted the number in bags.

[tobindax]

e.g. "You receive loot: Frostweave Clothx2" [245] (with 245 meaning that's the total now).

If it's possible to avoid an extra line the better.

[def9]

BagSync shows totals in your tooltip.

[red8981]

i think msbt has similar options, you can turn other stuff off if you just want to.

[Taryble]

Bagnon_Tooltip with Bagnon_Forever does this - you can get them from the Bagnon addon's folder, and install them without the rest of bagnon.

[Dainton]

Lootcounter seems to be the closest thing to what you're looking for. Not sure if it's still working or not.