The answer to your question doesn’t really depend on whether the technology is H.323 or SIP. There are two factors that ultimately determine how many users can be conferenced.
First, there is the hardware limitation of the MCU, or Multipoint Control Unit. Your VoIP system must have an MCU to support conferencing. The specs for the MCU will tell you how many users can participate in one conference call.
Second, there is the bandwidth limitation. Once you know what codec you are using, and it’s bandwidth, you can roughly gauge the potential number of users by figuring 3 times the codec bandwidth per user. In other words, if you are using G.729 codecs, which are 8kbps, you should figure 24kbps of bandwidth per user. Someone will scream at me for saying this. Cisco has a training example with the G.729 codec that comes out to slightly less than 11.5 kbps per user. I recommend you allow more slack than that, or you’ll end up with delays and drops.
So, how many users in a VoIP conference call? The lesser of these two numbers: the number of calls that can be supported by your bandwidth, or the number of calls that can be supported by your system’s MCU.
Have fun with your system!