From the Pew Hispanic Center:
The sharp decline in the U.S. unauthorized immigrant population that accompanied the Great Recession has bottomed out, and the number may be rising again. An estimated 11.7 million unauthorized immigrants were living in the U.S. in March 2012, according to a new preliminary Pew Research Center estimate. Different trends appear among the six states in which 60% of unauthorized immigrants live—California, Florida, Illinois, New Jersey, New York and Texas. Of these, only Texas had increases in its unauthorized immigrant population between 2007 and 2011. The analysis also finds that the post-2007 population dip was sharper for Mexicans than for unauthorized immigrants as a whole.
I, personally, have no idea how many illegal aliens there are in the U.S., and I doubt if the Pew organization is all that much more accurately informed. It's a tough methodological problem. On the other hand, I've found the Pew Hispanic Center researchers to be more honest than they need to be, so I wouldn't discount their numbers, either. For example, this announcement isn't welcome news for Schumer-Rubio.
One possibility is that they are off on the number but right about trends. For example, their claim of a falloff in new illegal immigrants after 2007 was backed up by the birth data. New illegal immigrants have a lot of babies, but overall Hispanic fertility dropped sharply in 2009-2010, backing up Pew's call of a decline in illegal immigration. We'll eventually see if the 2012 birth data backs them up on their call of an upswing.