64 KB per person per second. With a ten megabit connection, twenty people is enough to drown out all real traffic. With a ten megabyte connection, that number leaps up to 160. Either way, his actually getting enough people take down either of those two would have to be a hallucinogenic-drug-induced pipe dream.
(I forget which is used for measuring network speed. I believe it's megabits but I'm not sure.)
Megabits is more often advertised.
Either way, I doubt 20 people would be on the chat to even see that, even less who are willing to do something like that, even less who don't fear it's a secret virus to kill their computer, and even less who can do it properly.