As KelleyAnne pointed out nobody knows how many there are.
First define illegal. I’ve had co-workers working on a tourist visa, a student visa, etc. I’ve had co-workers where there are 5 or 10 consultants all using the same ID.
The anti-illegal voices agree that these are illegals. How do you count them?
Many in the 30+ H1b type visas lied, and their employers and client companies lied. We agree that they are illegals who are not being prosecuted.
These are people with visas we can track.
Then there are those with no Visa. How many of them are there? Many of these with no Visa are seasonal workers. They go South in November and return in March. So do we count the number in Summer or in Winter?
Why do we need an exact number anyway?
“First define illegal”
That is easy: Someone who is here in violation of the immigration law.
“Why do we need an exact number anyway?”
Have you heard anyone demanding the exact number when an exact number is impossible to identify?
It is that the number has been low balled in order to hide the severity of the problem.