How many DPAR's per LPAR on AIX?

Hi, We are in planning stages of moving 8500 ± users to (2) P550’s with 8 CPU’s each and 16 GB RAM per. We want an Active-Active (via Notes Clustering) configuration/setup. We are looking for recommendatons on DPAR’s per LPAR. I understand for optimal capabilities no more than 4 DPAR’s per PAR. If anyone has experience on this I’d appreciate your input and recommenndations.