They can ALL be rounded up by small forces through attrition, every last one of them. You don’t have to get all of them at one time; that’s the biggest straw man argument ever and the amnesty crowd keeps bringing it back again and again. And there is precedent for a roundup and deport effort. It was called Operation Wetback, and took place in the 1950s.
Sending the illegals back home involves about 2 months worth off normal border traffic.