Generally, if you go and research the numbers, the best guess for illegal residents in the US...without a visa or active Green Card....rests between 15 and 25 million people. Most reliable sources put the number closer to 20 million.
Eventually, it will come up how to reshape or solve this problem....as the wall is worked up and completed. So, I've put several hours of research and thought into this, and have come to this simple four-step plan (it's not rocket-science).
1. Open up a window for six to eight months. During this period, you go and register yourself at a local office for process which will eventually (figure four to five years to reach an end-point) get you full citizenship, and while waiting...get you a verifiable visa (an actual card that you need to carry with you), and the card will allow you to have a social security number and be fully employed.
2. This form you will fill out to enter the window-period? It'll ask you to bring/present an ID from our home country (license, passport, etc). No, that California driver's license won't work, nor will utility bills from your local electrical company. The form will ask you about former residence in your home country and ask five or six questions to ensure it's the 'right' story. Because.....we will go back and visit your home-town and ask to prove you exist. No trace, no ID, no desire to prove your ID? We don't process you, and we resettle you into a compound in Arizona or New Mexico where you will sit and work on getting your ID straighten out. As you can see...lying won't help you.
3. Prosecution or jail-time before? If it's a capital offense or more than a year in prison....you won't be processed, and you will be removed from the US.
4. Registered to vote in the US, and having voted at least one time? That will disqualify you entirely, and you will be removed from the US. No return, no waiver, and no second chance. For those who say it's unfair....on each registration sheet for any state, it says.....you certify yourself as an American citizen and "swear" to that affect. Obviously, you screwed up but that's your mess to clean up....not anyone else.
My best guess is that roughly 250,000 of these individuals who should go through and reach a residence visa....will not be able to do it because they have no ID or they have arrests in their background. Even giving them six months to go and ask for a new ID from their homeland.....probably won't be possible for various reasons. Those who've registered and voted? I'd take a guess that well over one million individuals have done so, and will not be able to stay. So we end up with roughly 18.75 million foreign folks registered and on a path.
You welcome them.....note that getting arrested and convicted of a capital crime over the next four years while they wait out this probation period....will suspend this process and probably end their chances.
End of the story. I know......we ought to have better programs and simplified ways of doing this....but it's the best you can do with the mess that you have.