Boris K
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One stack receives one dose of Armageddon, and 7 stacks receive 7 doses. Therefore, it is more beneficial for the one with 1 stack.
In general, I have never used Armageddon in a game against the computer. I used it once in the final battle in HotS. I was playing as Waishan or Iranna (I like these heroes equally), and my friend was playing as Deleb. Several enhanced Armageddons, and nothing remained of the demons.