Can someone give me a rough idea of what EC2 instance type I need for 2,000 concurrent users using Bitnami's cloud Openfire setup?
I looked and looked for a rule of thumb on this and didn't find anything.
I suggest you ask in the official forums of the application to get more information about this questions. They probably will give you a more accurate solution about it.
Please note that you can easily resize your server in the cloud once it's launched in case you need more resources in the future.