Looking at the output of the unix command 'top', you can see:
PID USER PRI NI SIZE RSS SHARE STAT LIB %CPU %MEM TIME COMMAND

18983 nobody 5 0 4564 4564 2848 S 0 4.7 0.4 0:52 httpd

18633 nobody 3 0 4552 4552 2904 S 0 2.0 0.4 0:56 httpd

The information in the "SIZE" and "SHARE" collumns are what we are most interested in. SIZE is the memory used by the apache process, while SHARE is the shard memory it uses. Based on that, fill in the information below. The total RAM on the server is the only other number you'll need. Ideally, this would be the total RAM installed in the system, minus what the operating system is using at any given time (which is often hard to figure out- ballpark for linux would be 64mb)

What is the max memory that Apache is using per process? (e.g. 4MB)
How much shared memory is Apache using per process? (e.g. 2MB)
How much physical RAM is on the server? (e.g. 256MB, or 1024MB for a gigabyte)

This is your ideal MaxClients setting for Apache.