Consider using Skype for large number conference calls.
TechCrunch » News Bits: Skype, Pandora and Meebo: "Skype is releasing version 2.5 beta of its client software on Wednesday. There are some cool new features: the ability to send and SMS to any worldwide cell phone, improved conference calling (you see who’s talking), and other small improvements.
Skype is also launching a preview of Skypecasts, a free new product that allows live, moderated Skype conversations of up to 100 people. The host may controls mute, eject and “microphone pass”. Skypecasts may be prescheduled and promoted via html snippets.
Skype is still lacking a record feature, and while there are some good third party products out there to bridge the gap, none seem to work on the new Intel Macs."