There are ten pies. An alien walks in and starts eating. When he's finished, someone comments by saying "he ate almost each pie".

What does this mean? That, lets say, initially there were 10 pies, and now there only two whole pies left? Or that the alien in question, ate a portion from each one of the ten pies, and in result we now have the leftovers from each one of the ten initial pies?

No native English speaker would say that. If there were only two out of ten pies less, a native English speaker would say He ate most of the pies or He at almost all of the pies. If he were trying to express the latter idea, that the majority of each pie was eaten, leaving ten separate leftovers, the native English speaker wouldn't say it this way at all. There's no simple expression for this; you'd have to say something like He ate most of each of the pies, and even then your listener would probably ask for clarification because the concept is so odd that your meaning would still be in question.

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SuStel
http://trimboli.name