[To get the exact value, first make a unit with this stats for the attacker:]
Combat: Damage base = 999
Combat: Damage number of dice = 1
Combat: Damage sides per die = 1
[Then make a unit with this stats as the damage taker:]
Stats: Hit Points = 10000
Now, the attack damage of the attacker will be equal to 1000 - 1000, meaning that all attacks will deal 1000 damage and will steal the same amount of HP, and you'll easily see how much it is.