With the Gran Turismo type in me, I mostly have put 16 to a track. I sometimes go for 20 to a track. Farthest I'll go is 24 cars to a track. One time on my former PC, I did Jacksonville Speedway with 40 or 43 cars to a track using the NSCR mod. I have Circuit de la Sarthe in my collection of locations. I don't know if I would try to put on anything north of 20 cars around Le Mans in my rFactor play. Even Le Mans 24 Hours- one of the best racing games of all time (to me)- had 20 cars around Le Mans, not to mention exclusively featuring the 2000 Le Mans field when you raced that event. It's something I would surely try if I were ambitious enough. Even more ambitious, I'd invite only GT cars around any of the tracks that use the Nürburgring Nordschleife.
And yes- I too am disappointed with the sounds of the Aston Martin DBR-1. Otherwise, this car is pretty beast to drive.
If I were to suggest some cars for a hypothetical EnduRacers Endurance Series SP4 pack, this would be my list of cars to consider: Audi R8 GT3, Audi R8 LMS, BMW Z4 GT, Chevolet Corvette C7.R, SRT Viper, NISMO GT-R GT3, Audi R18 TDI, Audi e-Tron, Lola B12/60, Lola B12/80, Toyota TS030, Toyota TS040, Porsche 919 Hybrid, and yes- I'd even include this: the DeltaWing. Basically all or most of the more recent GT and prototype machines are what I featured here.