If you want Ansys Polyflow to perform the decomposition before it begins the calculation, but you want to specify the number of sub-parts yourself, you will need to make sure that you have selected the classic direct solver (see Selecting the Solver) and then start Ansys Polyflow with the -opt number_of_sub-parts command line option (where number_of_sub-parts is the desired number of sub-parts, such as 8):

 polyflow -opt 8 < my.dat 

or

 polyflow -opt 8 < my.dat > my.lst 

See About Domain Decomposition for more information about sub-parts.