Hello,
This really can't be done using this strategy, and there shouldn't be a reason to do so either. The server has no concept of the number of threads running in soapUI, only the amount of transactions it has to handle. The FixedRate Strategy lets you set the desired TPS, and soapUI will create new threads to keep up with that demand. If more Threads were started then they would just sit there waiting, doing nothing at all.
If you really want soapUI to run 300 threads, you could use the Simple Thread Strategy and manually set 300 Threads. This would give you a MUCH higher TPS than 50 though, so you would also have to add a delay step in your TestCase. Doing this no longer guarantees 50 TPS. By changing the delay time, you can get close to 50 TPS (the exact delay time required depends on the time the actual requests take, and can even vary for each run). What exactly is your reason for wanting 300 Threads?
Regards,
Dain
eviware.com