The easy way is to start all the scripts from the command line at the same time and have each script programmed with number of users, etc. I use Windows, and create a batch file containing multiple lines like:
start /REALTIME java -jar ApacheJMeter.jar -n -t test_script1.jmx -l results1.jtl -j log1.log -Dthreads=40 -Dduration=1800
However, for the scale of your setup, I would recommend you look into a better method of controlling your testing through automation.
I have set up a Jenkins server, which is able to run JMeter scripts using maven, or shell scripts, and then aggregate results, create graphs, etc. It is able to trigger jobs in parallel, sequence, randomly, or triggered by other events.
I would also look at setting up your scripts to use properties for number of threads, loops and duration, so you can control them easily from Jenkins without modifying the scripts. Create a User Defined Variables config element, and copy property values to variables, setting defaults, ie:
THREADS ${__P("threads", 25)}
This will mean ${THREADS} can be used in the thread group, and will be default value 25. If you assign a value on the command line it will be used instead of default. You can define it on the JVM command line, when you start JMeter like this:
-Dthreads=40
In your script, the value of ${THREADS} will now be 40.
