The default is set or configured in your httpd.conf file. In most cases, its a good idea to have more than one running at boot or start time. This brings better performance.
My small server I have 8 startup by default which is a reasonable amount for a small server with not much traffic.
Check your httpd.conf file though, it is set there. If you make changes, just restart your server. If you have small amounts of traffic, 12 isn't bad to start off with.