There is a place in jMeter where you can specify the number of users you want to start concurrently. I assume you can't put a number where it exceeds the limitation on your hardware & OS.
If you set the value to 0, then JMeter will immediately start all of your users.
But given a specific hardware and OS, let say windows 7, i5 CPU, 8GB RAM. How do I know or calculate the number of users I can put to start all the users concurrently? Although I can trial & error to find out the numbers, I hoping if there is a way to pre-calculate it.