For each presentation... However the latter statistic is interesting as well!
If you go to the iMoot lounge and check the participatnbs, the curent number is 252.
AFAIK BBB is limited to 50 or 60 people anyway, unless the organisers have got something special set up.
In some ways I'm glad UNSW has Blackboard Collaborate. Also our program uses Cisco WebEx. Both are excellent. But then again not free.... I will run a exam preparation workshop tomorrow using webex. It will be interesting to compare them again. Btw I had between 12 and 20 I think. Not bad considering I'm in higher Ed which is not relevant to the majority of Moodle users...