If you need to simulate a load on an Apache server (or any web server actually), you can use Apache Bench, which is included in the standard Apache HTTPd distribution. This tool will launch connections to your webserver as instructed to simulate multiple users and will help you to tune your Apache settings.
You can find the synopsis at the Apache website. Most common options are :
* -n : number of requests to perform
* -c : number of concurrent requests
Other options allow you to control precisely the request to send, proxy settings, user authentication, cookies and much more.