Change the thread count of test plan in JMeter, at run time

Question

I want to change the number of threads for a JMeter test plan at runtime.

I have Googled my problem and found a proposed solution to use JMeter plugins. But in this

Answer 1

IMHO that's just a fancy feature that has no real benefit when doing proper performance testing. In order to generate relevant test output (report), you need repeatability, and clearly defined test methodology and scenarios. In order to compare impact of any application/server/infrastructure changes, you need repeatability.

What do you mean by

We can't predict the user of our site

That's why we do performance testing at the first place. To find out what is our application/infrastructure limit.
I.e. the most significant metric you can produce is how your application response time changes when number of parallel users change. But not change erratically, in run time.

With jMeter plugins' Ultimate thread group you can cover any imaginable scenario.