According to ESPN Stats & Info only 3 perfect brackets remain on their website out of the 17.2 million submitted into the ESPN Tournament Challenge. Via Reddit there are an additional 2 perfect bracket's on Yahoo, one on NCAA.com, one on Politico, and one on CBS.com for a total of 8 perfect brackets remaining on all platforms.
All 8 of the brackets have Michigan, Michigan State, and Gonzaga heading to the Sweet 16.