I have done 4 (8 pounds each); I have seen folks on this site do 6 and Chris Lilly did 8.
all using the main and upper grates.
Put them on at 5-6pm eve prior to party.
They'll come off at about 11-1 next day, then you can wrap in foil, towels and put in cooler to hold til you need to pull them.
Pull them as you need to serve, they will stay nice and hot for you.