>>17552
I can't remember the exact location, but I believe it is something like this: You need enough mages to protect the soldiers from other mages. You also need enough soldiers, or the enemy soldiers will just come kill your mages while they're locked in a wizards duel.

>>17553
Found it!

“Abandoning the subject, Oromis asked, “How would you kill with magic?”
“I’ve done it many ways,” said Eragon. “I’ve hunted with a pebble—moving and aiming it with magic—as well as using the word jierda to break Urgals’ legs and necks. Once, with thrysta, I stopped a man’s heart.”
“There are more efficient methods,” revealed Oromis. “What does it take to kill a man, Eragon? A sword through the chest? A broken neck? The loss of blood? All it takes is for a single artery in the brain to be pinched off, or for certain nerves to be severed. With the right spell, you could obliterate an army.”
“I should have thought of that in Farthen Dûr,” said Eragon, disgusted with himself. Not just Farthen Dûr either, but also when the Kull chased us from the Hadarac Desert. “Again, why didn’t Brom teach me this?”
“Because he did not expect you to face an army for months or years to come; it is not a tool given to untested Riders.”
“If it’s so easy to kill people, though, what’s the point of us or Galbatorix raising an army?”
“To be succinct, tactics. Magicians are vulnerable to physical attack when they are embroiled in their mental struggles. Therefore, they need warriors to protect them. And the warriors must be shielded, at least in part, from magical attacks, else they would be slain within minutes. These limitations mean that when armies confront one another, their magicians are scattered throughout the bulk of their forces, close to the edge but not so close as to be in danger. The magicians on both sides open their minds and attempt to sense if anyone is using or is about to use magic. Since their enemies might be beyond their mental reach, magicians also erect wards around themselves and their warriors to stop or lessen long-range attacks, such as a pebble sent flying toward their head from a mile away.”
“Surely one man can’t defend an entire army,” said Eragon.
“Not alone, but with enough magicians, you can provide a reasonable amount of protection. The greatest danger in this sort of conflict is that a clever magician may think of a unique attack that can bypass your wards without tripping them. That itself could be enough to decide a battle.
“Also,” said Oromis, “you must keep in mind that the ability to use magic is exceedingly rare among the races. We elves are no exception, although we have a greater allotment of spellweavers than most, as a result of oaths we bound ourselves with centuries ago. The majority of those blessed with magic have little or no appreciable talent; they struggle to heal even so much as a bruise.”

why would you care about anything in those trashpiles?