Based on data from the NOAA Storm Prediction Center, there have been 46 tornado reports in Illinois in 2010, as of September 14. These reports are based on damage at a particular site. The real number of tornadoes is often less than the first reports, as further analysis may show that the same tornado produced damage in two or more locations.
Of course, the 2010 tornado season is not over yet. The climatology of tornadoes in Illinois shows that it is possible to have tornado activity somewhere in the state all the way through December.