Cote,
it is a while ago now, and I don't have access to a BW 7 system anymore, but I did figure out what needed to be done in the end.
There is a transaction called RZ12. Here you can define server groups, and also restrict how much resource that they take.
In the APD you define which server group you want to use when running process in parallel.
I think I would advise experimenting with this in a test environment, and find out which settings restrict the number of processes that the APD starts up to the optimum level.