Autoscaling won't help with scaling bbb server. That's just not how BigBlueButton scales. Instead, you'll have to code your own stuff which orders/starts more bbb servers as required. You can use for example ansible roles to set up those servers. Then when your servers are idle again and there is no one using that server any more, throw that server away.
Just adding more CPU cores,... does not work because the event processing mostly runs single-threaded and will reach its limit at usually something between 150 to 400 users depending on used hardware and setup + configuration.