Where are you getting this number from? If we had a barrier that would be impossible to cross without some major engineering, along with motion sensors, then we could guard the border quite well even with the numbers we have (about 9000). Tripling that number (which could be done with very little cost, comparatively speaking), would make it that much more secure.
If you want to secure the US border, the way that you want to, you need boots on the ground. This is not Hollywood
Of course, no one said it was going to be easy! It van be done. Uneven topography and all. Bulldozers move mountains. if Boulder dam can be built with the technology of the 30's the border can be controlled with the technology of the 21st century. Building a wall/fence CAN BE DONE.
The knowledge that it exists with landmines between the wall/fences is a deterrent.