The Distribution of main test runs across tasks option on the Distributed tab determines the distribution of Main Test iterations into tasks that the workers perform. The calculation is based on the total number of iterations your test contains.
By default, the Default option is selected and the text in parentheses shows the number of iterations per task and number of tasks. The default is calculated by dividing the number of iterations in your test by 32 (an approximation based on a setup of 8 workers, with a target of 4 tasks per worker), and using the closest number to that. For example, if your test has 90 iterations, the default will be 3 iterations per task and 30 tasks, as shown below.
If you run your test and this does not seem efficient, you can change the number of iterations per task and number of tasks. To change it:
Select the second option. The number field becomes editable.
Enter the number of iterations per task you want to use.
The number of tasks is then calculated (total number of iterations divided by number of iterations per task) and shown in parentheses. For example, if you had the same test with 90 iterations, but changed iterations per task to 6, you get 15 tasks.