Hi,
There are a couple of ways to change the number of test sockets depending on whether you would like to do this at edit-time or run-time.
If you just want to change it in the TestStand environment, you can go to Configure>>Model Options, and there is an option to change the number of test sockets. Note that this option will only be available if the process model is set to parallel or batch.
If you want to setit programmatically, you would have to override the ModelOptions callback, as described in this article:
http://www.ni.com/white-paper/2802/en/
Depending on your version of TestStand, some of the images/steps in the above article may not apply, but the general steps are still valid.
Regards,