Who is to determine how many agents are needed? The border patrol? That's a bit self serving to allow a bunch of union bureaucrats make that call. They certainly seem alert enough to know how many have entered the country. If they can count 'em, how come they can't stop 'em?
If we truly want to patrol our borders and keep them secure, common sense would tell us that we need enough agents to patrol the border 24 x 7, with the number of agents on each shift to be able to see any part of the border to which they have been assigned with binoculars (or whatever other equipment they might have). It is ridiculous to expect a border patrol agent to be responsible for miles and miles of border that he cannot possibly be watching all at the same time.
As to who makes the decision how many, it would be whoever is making that decision now. But of course, it would have to be funded. And it would be expensive.