It's never mentioned. At few hundred, at least, because the book has history from Panem going back over 75 years (the 75th year of the Games and pre-first-rebellion, when the Games were established). It's more than 75 because there would have to be a hundred or so for Panem to develop into the bloodthirsty country that it is when the stories take place. But there's really not a specific number, it's left to the imagination.
