What is happening here is due to a 4Gb memory limit for 32 bit programs. When AMS was in development the sim would crash because too many cars were being loaded, exceeding the memory limit.
This was eventually solved by optimizing the cars and also setting a limit on the number of cars AMS would load depending on the car type. Some cars that are graphics heavy have a limit of 20 or so, other lighter cars can be up to 32.
The message was introduced early in the dev period to let users know that they were getting close to crashing the application and reduce the car number accordingly.
This allowable car limit is also track dependent too... very heavy tracks take more memory so that leaves less for the cars to use.
Another consideration is replays saved to memory, or motec telemetry that is saved to memory. The message can be helpful just in case you get close to running out of memory and prevent you from crashing the sim.
If you see this message and all is well then you should be ok, but you are very close to the 4Gb limit.
Cheers