Let me add a bit of interesting information here: there can be up to 8 different sprites (or animations) for items that have different sprites for stacks. The amounts in which they change are:
Not all items have different sprites for all amounts, however, as it's the case with Phoenix Eggs (it has sprites for 1, 2, 3-4, 5-9, 10-100). When this happens, the client simply repeats a given sprite. This is how the sprites for Phoenix Eggs are organized:
The reason there are 4 rows is that each amount is also independently animated with 4 frames. This is actually quite uncommon, I can only recall the enchanted gems also having animated sprites and different sprites for amounts.