Only once. They just drive a remote controlled truck through to set off the mines, and then the rest of the horde walks through the cleared path.
The Israelis say their fence works just fine.
I’m sure one could put enough explosives along the border to deflect whatever the invaders try. To top it off, place snipers every 100 yards. Maybe have automatic gatling guns every 10 yards. Maybe have a deep and wide trench filled with sulphuric acid. Have thousands of rabid dogs roaming between two parallel lines of high concertina wire. Have all those. Whatever it takes. God knows if we don’t spend our money to defense the country and it’s borders, the LIBs will create “programs” to help illegals get here and vote for them forever.