Conferencing: maximum number of users?

What’s the maximum number of callers can talk in one conference? Is it possible to make one room where 30 people can talk and hear each other. What hardware do i need for such * server if the conferencing is the only feature of that server? What is difference if some callers are from IP network (SIP or H323) and some from Zap? More Zap - more power or otherwise?

Thanks in advance.