Testfor @e stacked items count as 1

@e returns the number of entities.
This is most likely intended, as multiple stacked item drops are still one entity.

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Agreed.

I think this is a bug. While it is true that, technically, stacked items are one entity, the selectors are designed to be useful. Counting 10 stone as one if they are close together but as 10 if they are far apart is not useful. The commands are allowed to deviate from technical accuracy when it is deemed necessary. For example, the summon command can summon a lightning bolt even though a lightning bolt is not technically an entity.

Dropped items stacking is just a method to reduce lag. It shouldn't affect commands.