CBP apprehended 60k+ last month, and that's just at ports of entry. That equates to 720K per year, so millions will not take that long.
The math is very simple. If it takes CBP XXX number of agents to patrol a completely open stretch of border, it goes without saying that having a huge wall to help prevent crossings should mean less of a need for that same number of agents. Same goes for drone sweeps, air sweeps, etc. All can be reduced in places where the wall is in place, and redirected to make the ports of entry even stronger.