A very common practice in JMeter is to make the number of threads per server configurable by passing in as a parameter to your thread group. We recently received a question from RedLine13 customer University of California about this. Sundar Thiru, Manager of Performance and Automation Testing, asked how to configure JMeter thread counts per test in RedLine13.
You simply launch this in RedLine13 and use Advanced JMeter Options to pass in your settings at runtime.
Once you have a good test you can just clone the same test configuration and change the size.
This is done in advanced cloud options.
If you change the size of the test you might want to change -JNUM_OF_USERS to take advantage of larger size instances.