The asterisks indicate that the team takes part in the full season of Intercontinental GT Challenge, the rest are there just for this race only.
As for number of drivers, there are different regulations for different classes within the race. Important - the classes are defined solely by driver categorisation while the cars are equal in terms of capability. Pro teams are allowed 3 drivers (no additional rules on categorisation), Silver teams are allowed up to 4 drivers (all Silver), in Pro-Am you can have a maximum of 2 Platinum and 2 Bronze drivers, and finally in Am you're allowed 4 drivers, with a maximum of 1 Silver driver - so either 4 Bronze, or 3 Bronze + 1 Silver.
My favourite Pro driver is Kevin Estre, so I'll be rooting for car #21 (Rutronik Racing) to get the overall win. In Pro-Am, I'm always cheering for Herberth Motorsport (car #911) as I got to meet the Renauer brothers a long time ago, love the familial atmosphere in the team and have been following them ever since.