How do I decide on placement of biological indicators within an isolator or autoclave

I would like to understand the criteria for placement of Biological indicators for measuring the sterilization effectiveness in an isolator and an autoclave. What is the basis of deciding on the location of the BI and what is the minimum number of BIs to be placed per given volume?

There is an european regulation about the number of indicators, at least 5 for big autoclaves, merck indicators on their instructions gives the reference