Hello Madhavisri,
The official system requirements are listed on the
System Requirements page.
When interacting with a tested application, TestComplete gets a lot of data from the application process, builds and keeps up-to-date the application object tree and spend resources for other housekeeping. Besides of this, the tested application also consumes resources in order to perform its usual tasks. Moreover, the virtual machine emulation software consumes resources as well.
So, it is quite expected that a single machine cannot easily handle 8 such environment at a moment. You need to get a much more powerful machine or spread your tests over several host computers.