Indicators for right balance of the number of DB2 instances per AIX LPAR
You won't get any specific answer to this question, because as you observed, it depends.

The answer depends on your company's tolerance for having one DB2 instance adversely impact the performance of another's.  You may be able to control or limit this impact if your AIX admins are willing to learn and set up WLM policies at the operating system level (not DB2 WLM).

This is really a question of capacity management.  Understanding the workloads in your environment is key here, and being able to identify (or predict) when you'll run into limits will influence your decisions about how many instance(s) to put on a single server.