Recently I stumbled over a handy perl script, which analyzes your currently running apache clients and helps you with tweaking your MPM settings based on calculations:
The script outputs looks like this:
Total memory available: 7.80G
Total used by apache2 (97 instances): 2.66G
Total used by other processes: 2.57G
Average memory used per apache2 process: 28.03M
Recommended number of processes based on Average: 191
Needed memory for 500 processes based on Average: 13.69G
Max memory used for apache2 process: 45.22M
Recommended number of processes based on Max: 118
Needed memory for 500 processes based on Max: 22.08G
Mean plus two Standard Deviations (bulk of usage under max): 43.11M
Recommended number of processes based on Mean + 2*Stdev: 124
Needed memory for 500 processes based on Mean + 2*Stdev: 21.05G