I prefer the term "barrier", which could be a wall in places, or something else. It's pretty obvious that any time one wishes to secure an area, any area, you can do it with X men, or many fewer with a suitable barrier. In places it can be just steel poles, as we see near protected facilities, to prevent vehicles from traversing.
There is a Fed Reserve Bank near me we walk by often, their security is interesting and I'm sure I see only parts of it. They have a nice museum inside one can visit and watch pallets of money going by, we're told each pallet contains $5 million and up, on automated "fork lifts" (behind glass of course). So, inside that bank muct be billions of dollars in currency. (Not to mention trillions of non-paper assets).
It's a neat visit if you're here and have an hour to spare.
So, we could "secure" the southern border with millions of men, or we could do it with thousands and a suitable barrier depending on terrain.
Another issue is over stayed visas of course. This is much harder to do in the Schengen Zone for obvious reasons.
