What skills should Necromancers take when facing Orcs on a very large map?

The effect depends on the quantity, just like the amount of health in a unit. It doesn't depend on the number of rage points, but on the number of rage points removed. The manual states:

Lost_Rage_Points = 1100 * Absorbed_Damage / Unit_Health

Let's consider an example with a unit of 70 Executioners. The health of one Executioner (without modifiers) = 40. 40 * 70 = 2800 - the total health of the unit. This is a standard situation where the Barbarian has Blood Rage at an expert level, meaning 70% of the damage is absorbed. Our Necromancer is level 25. This is quite realistic and achievable. Therefore, the Earth elementals will deal 75 damage. 70% of this is absorbed, so the absorbed damage = 52 damage (approximately). Next, we plug this into the formula:
And this is with 70 Executioners, which is not the limit by the end of the game. Although, of course, everyone can decide for themselves how accurately I have matched the number of units, the hero's level (although, plus or minus a few damage units shouldn't make much difference), and whether this is a lot of rage points or not. :)