Well I don't know what to make of the whole story. Aside from the debatable issue of whether flushing is even a good idea, the discrepancy in fluid amount is not readily explainable.
But the precise answer to your question is 8 liters capacity. Use Dexron II or III
Maybe they spilled the other 2?
Are you taking the car to one of those big chain operations, like a Rapido-Lube thingie?
I suggest this because some of them (not all) have a habit of over-selling product.