There is a documentary film that was released on this very subject about 10 years ago and it goes a great deal farther than most would believe because there was a great deal that was not disclosed for many years.
The actual figure is "guesstimated" at somewhere over 2,500 according to that documentary.
Some of the locations are actually quite surprising. Of particular interest for many would be "Operation Gas Buggy" which was used as a precursor to "fracking" and that one did not work either. It fractured the shale perfectly to extract the natural gas, but the gas was so toxic with radiation the end product was not usable.