CIS, Numbers USA, and FAIR - which are the three top Conservative immigration think tanks - all use that number.
No doubt there are flaws in that number, but the consensus seems to be that it is the best one available.
Additionally, the census numbers of population decline in Mexico also support the number of 16-20 million Mexicans moving to El Norte illegally.
Similar demographics in South and Central America support another 10 million of their collective populations moving to the US illegally.
SO figure 12 million here in 1996 plus 16-20 million Mexicans plus another 10 million Non Mexican South and Central Americans and you get an easy 30 - 40 million illegals in the US.
This is consistent with what we see in our new Press 1 for Spanish, Press 2 for English culture in the US.
Arrest all illegals, seize all property they possess and deport them.