Essential Thread Group configuration would be:
- Number of Threads:
300
- Loops: tick "Forever" or type
-1
- Duration (seconds):
300

However it is recommended to increase (and decrease) the load gradually to both JMeter and application under test could adopt for increasing load. Moreover you will be able to correlate increasing load with main performance metrics like how does response time and/or throughput change as load increase, when errors start occurring, etc.
So it will be more convenient to use i.e. Ultimate Thread Group where you can easily configure workload like:
- During 1st minute users arrive
- Then 300 users hold load for 3 minutes
- Then during last minute users are being shut down

You can install Ultimate Thread Group (as well as other Custom Thread Groups) using JMeter Plugins Manager
