You're using Apache 1.3, or Apache 2.0 with the prefork MPM, and you want to tune MinSpareServers and MaxSpareServers to the best settings for your web site.
Will vary from one site to another. You'll need to watch traffic on your site and decide accordingly.
The MinSpareServers and MaxSpareServers directives control the size of the server pool, so that incoming requests will always have a child process waiting to serve them. In particular, if there are fewer than MinSpareServers idle processes, Apache will create more processes until that minimum is reached. Similarly, if there are ever more than MaxSpareServers processes, Apache will kill off processes until there are fewer than that maximum. These things will happen as the site traffic fluctuates on a normal day.
The best values for these directives for your particular site depends on the amount and the rate at which traffic fluctuates. If your site is prone to large spikes in traffic, MinSpareServers needs to be large enough to absorb those spikes. The idea is to never have a situation where requests come in to your site, and there are no idle server processes waiting to handle the request. If traffic patterns on your site are fairly smooth curves with no abrupt spikes, the default values may be sufficient.
The best way to watch exactly how much load there is on your server is by looking at the server-status handler output. (See Recipe 11.4.)
You should also set MaxClients to a value such that you don't run out of server resources during heavy server loads. For example, if your average Apache process consumes 2 MB of memory and you have a total of 256 MB of RAM available, allowing a little bit of memory for other processes, you probably don't want to set MaxClients any higher than about 120. If you run out of RAM and start using swap space, your server performance will abruptly go downhill and will not recover until you are no longer using swap. You can watch memory usage by running a program such as top, which shows running processes and how much memory each is using.