My OCD kicked in and I felt compelled to count how many units I lost at the hands of Atlantis and Patala on turn 20. We lost 588 undead, or about a third of all my units in 8 separate battles -- only two of which I won (one of which was against an undefended province). That was a bad turn, although worse was to come.
Anyway, I would have had 1725 units in my empire as a best case scenario.
If I had somehow been able to put all undead into a single large horde, Atlantis would have utterly destroyed it with his scary super-soldiers. If you're curious, here is what most of my battles with Atlantis looked like at around that time:
