I think the most I've ever had on telecon was about 30, but I've had 55+ on VTC... We had so many cameras on the net that I couldn't see everyone at once because our giant TV screen only divided up 16 ways. Add in about a 1 second delay, and it was a mess trying to keep things in order. I had to tell people to raise their hand if they had a question so I could call on them, otherwise, several people would be trying to talk over each other.
We have one guy (our lead Lockheed engineer) who was always overly polite on VTC and telecons... People would talk at the same time as him (which naturally happens) and he would always insist that they go ahead... which was unfortunate, because they would usually say something silly or incorrect, and he would have to correct them anyway. He was one smart dude.