Entry's confirmed:
Prototype (P) - 16
Prototype Challenge (PC) - 8
GT Le Mans (GTLM) - 10
GT Daytona (GTD) - 19
Total Entries - 53
(Detailed List - PDF)
(Looks like 12-13 will be sharing boxes with somebody, always makes for some interesting pit action)
http://sportscar365.com/imsa/tusc/roar-before-the-24-entry-list-revealed/