For a typical smaller server configuration, with Prefork MPM, you probably want something like:
Note, this puts the limit at 500. If you want more than that just increase the value. The values are sensible for generic small websites, but if you're site isn't "generic" you really need to take some time and tailor these values to your site.
If you're getting much larger than this, it's time to switch to a different MPM, like worker or event. This may require changing various aspects of the site however, depending on what technologies you're using.
For a Threaded MPM (Event, Worker) and a generic smaller website:
MPM WinNT only has a single process (child), so you'd want to bump the ThreadsPerChild to something sensible. Another answer has links to the relevant Apache man pages.