Social Security says that there are 9.5 million "extra" filings for folks who don't really exist (illegals.)
So right there, you have almost ten million illegal aliens working in the above-board economy, where employers file.
Add at least another five to ten million who work for cash under the table.
Add their wives, girlfriends, and many children.
Do the math.
I know that at our local (San Diego) public schools, there are 35 to 40 kids per class, and HALF DO NOT SPEAK ENGLISH as their first language. IOW, they arrived sometime AFTER their birth, because at age ten and up, they still don't speak English. IOW, they are newly arriving illegal aliens.