Define 'all' please. All the ones in Vegas, all the ones in the US, all the ones in the world? The dominant companies are ITT Corp, Hilton Hotels, MGM-Grand and Mirage. The top tier is more or less an oligopoly between these four; however, independent smaller casinos still survive, mostly in Atlantic City and on the riverboats. So the basic answer is "between 4 and 6 at any given time for the really big ones."
(Most Indian casinos are actually contracted from one of the big players.)