General Discussion

There's simply no comparison. It's not even apples and oranges. It's like apples and hand grenades.

12 million is a scale that dwarfs anything he did in his first term.

The consensus is that it's not logistically possible. The infrastructure doesn't exist and would have to be built on a massive scale. The government has no clue who most of them are or where they live. Hiring would need to be done on a massive scale, probably at least tens of thousands. Coordination with the countries to which they're being repatriated will be extremely complicated, and it's very likely some won't cooperate. Only eight of the top ten have any kind of agreement with the US to do so: Mexico and Venezuela. For the other eight (and any others), repatriation would need to be handled on a case-by-case basis.

Aside all that, the scale is almost certainly unworkable.

It would be the equivalent of deporting the entirety of New York City and the entirety of Los Angeles.

Or the equivalent of deporting the entire populations of:
Chicago
Houston
Phoenix
Philly
San Antonio
San Diego
and Dallas, seven of the top ten cities in population in the US.