We recommend your servers have at least 4GB of physical RAM. A high number of concurrent users means that a lot of RAM will be consumed. You usually don't need to assign more than 4GB per JVM process, but can fine tune the settings as required.
You should also not run any additional applications (other than core operating system services) on the same servers as Confluence. Running Confluence, JIRA and Bamboo on a dedicated Atlassian software server works well for small installations but is discouraged when running at scale.
Confluence Data Center can be run successfully on virtual machines. If you're using multicast, you can't run Confluence Data Center in Amazon Web Services (AWS) environments as AWS doesn't currently support multicast traffic.
Add-on licenses for Data Center are sold at the single server rate, but must match or exceed your Confluence Data Center license tier. For example, if you are looking to have 3,000 people using Confluence Data Center, then you would buy any add-ons at the 2-001-10,000 user tier.