I know, how to send multiple message: you have to add people as your buddies, then scroll down the page, when you are on: private messages page. In the bottom is: PM to buddies, choose your buddies and compose a message.
But I dont know, how many people you can add as buddies
You're wecome.
But maybe there is some other way to do that, you would have to add so many people as buddies...
I dont know, how many you can add, never did it.