If you want to conference your calls, you must buy a license. This is not available in the free edition. While there are many paid-for conference call options, VoIP providers offer them Skype, GoToMeeting, and many others. However, it's easier and cheaper to do your own through 3CX.
You are limited to 32 callers in total, using up to 8 conference calls on your system. The license you buy will determine how many you can have. Buy your license with this in mind if you have the need for a lot of conference calls.
Conferencing is enabled by default with an extension number of 700, allowing 4 conferences to be going on at the same time. You can change these settings from the Settings | Advanced Settings | Conferencing tab.
The more simultaneous conferences you have, the more processing power you will need. This is not something you'd want to do on a virtual PBX system. I...