MaxClients defines how many simultaneous requests can be served. Any connection requests from browsers that come in after that will be queued. Default hard limit of 256.
Is ok to set high for static content, but bad for rendered pages.

MaxSpareServers
MinSpareServers
StartServers
ServerLimit

Thrashing where the system is just swapping pages from physical memory to virtual memory (on disk), and vice versa, without doing any real work.
Apache processes with modules (mod_perl, mod_python, mod_php) can easily be 21MB per process.
Can improve with PHP opcode cache/accelerator, then you can make each Apache process take as little as 12MB.
this is where one_liner for RSS sort processes by mem usage

# ps auwxx -- sort rss
# icps "semiphores"

if apache is high mem usage, check MaxRequestsPerChild and keepalives