I don't think anyone really knows how many illegals are in the U.S., or how many are coming in each day. The Federal government seems to have no way of knowing, so I doubt any state or other organization knows either. It could be much higher or much lower than what we hear in the media.
This study was published in Jan. 2005 and obviously drew from data obtained during 2004. The number derived by methods in this study - 21 million - would almost certainly be above 25 million by now I'd think.