How to run the JMeter for more than 300 users?

Run in Non GUI distributed mode to run for more than 300 users. It depends on RAM size. For the system with 3.2 GB, you can run up-to 1000 users.

Step1:
Configure Jmeter for Distributed testing

Step2:
Place your script and csv in /bin folder of the jmeter.

Step3:
Open command prompt , go to jmeter bin directory and give the following command

jmeter -n -t testplan.jmx -R ip1,ip2 -l resultfilename

How to handle sessions in JMeter?

To handle the sessions in JMeter you need to do the following steps:

Add->Pre-Processors->HTTP URL Re-writting Modifier for the sampler where your session variable is present

Then add your variable in session argument name and check chache session id.