[The Might]

In the books Eomer does not have 2000 men like Aragorn says in the movie, but only his eored of 120 men.

"In times of war or unquiet each Marshal of the Mark had under his immediate orders, as part of his "household" (that is, quartered under arms at his residence) an eored ready for battle which he could use in an emergency at his own discretion. This was what Eomer had in fact done; but the charge against him, urged by Grima, was that the King had in this case forbidden him to take any of the still uncommitted forces of the East-mark from Edoras, which was insufficiently defended; that he knew of the disaster of the Fords of Isen and the death of Theodred before he pursued the Orcs into the remote Wold; and that he had also against general orders allowed strangers to go free, and had even lent them horses" (UT).

I also thought that the size of the army Eomer left with was a bit exagerated, and that he wouldn't have been allowed to do such a thing.