Hi Mohamed,
Yes, if the the number of DIA processes in your system is 20 you can't set PARALLELSCRIPT to more than 16.
Process you described works for BGD, but not for DIA. For DIA is there is no available process it's a failure.
Usually Basis can increase number of DIA up to some level and then you can see what is memory and CPU usage. I'd guess you doing this in your Dev box; does Prod have same horsepower?
Regards,
Gersh