The number of simultaneous ExtendSim instances that can be launched is limited to the number instances you have purchased. As long as your Maintenance & Support Plan is current, by default, you can launch up to 4 instances of ExtendSim. However, you can purchase the ability to launch up to 8 or 32 instances.
From a computational perspective, it makes sense to keep the number of ExtendSim instances you want to launch equal to the number of cores on your machine.
If you have 8 cores on your device, it is possible to launch up to 32 instances if you purchase that ability. Keep in mind, that as the number of parallel instances of ExtendSim grows and the number of models open, running, and getting saved grows, you're consuming more RAM and more hard drive space. Therefore, it is important to have enough hard drive space and RAM to accommodate your parallel computing goals.
Note: Each instance of ExtendSim runs independently of the other ExtendSim instances. And those instances will be manage by your Windows operating system.