Welcome to the forums. I would say 4 ~ 6 dNaNo's would be a good. The Mini-96 is good size in length, so the cars should get spread out a bit after the start of a race. You can always open up the turns if needed by adding expansion tiles that bring the total lane width to 30 1/4" from inside to inside border. Open up the lanes all the way around and you can most likely get 8 ~ 10 dNaNo's on the track.
These guys got a lot more dNaNo's on an RCP with 50cm tiles. This track in the video has a lane width of 33" from inside to inside border excluding the first 3 tile wide straight away, while the Mini-96 with expansion tiles has a lane width of 30 1/4"
http://www.youtube.com/watch?v=Qm0QvD4Uodk&NR=1