The amount lost depends on the number of units, just like the amount of health lost by a unit.
1100 * 52 / 2800 = 20 units of rage lost.
And that's even with 70 Executioners – which isn't the limit by the end of the game. Although, of course, everyone can decide for themselves how accurately I compared the number of troops, the hero's level (although, plus or minus a few damage units shouldn't make much difference), and whether that's a lot of rage units or not :)
But here, the point isn't how many rage points the Fire Fields will take away (even if it's only 1), but that they will reduce the rage to the developed Blood Wrath, and the attacking stack will deal 40% damage instead of 30%.
Творят настолько черные дела,
Что до таких блистательных идей
Не вдруг дойдет и записной Злодей
Темный адепт клана GODS