Unable to perform parallel execution on safari on mac using selenium

I have around 1600 java TCs written using selenium. While executing test scripts in parallel (say thread count = 5), I am able to trigger 5 safari webdrivers and scripts are executing parallel.

But, when the 6th (threadcount +1) script gets invoked, it is throwing below exception:

Could not create a session: The Safari instance is already paired with another WebDriver session.

I am quitting and closing the webdriver session after each test case. The same piece of code is working fine for other browsers except safari.It should run on the same machine with 5 instances running parallel.