For a full list of what's supported in which version of JIRA, check out this list here. There's a similar list for Confluence.
Specifically about Oracle 12c release 2, yes you can use that release of Oracle with the latest JIRA and Confluence versions. I would strongly recommend using the release 2 driver for JIRA and Confluence (18.104.22.168) which you can obtain here from Oracle. The ojdbc8.jar file needs to go into the lib folder in your application install directory, and any older ojdbc8.jar/ojdbc7.jar files should be removed from that directory. The release 1 drivers had bugs that affected JIRA.
You'll want to take a look at the JIRA Sizing Guide. 2000 active users would constitute an Enterprise scale instance.
This should be a top of the line server with enterprise grade processing power (e.g. 6 quad cores or more) and the highest possible I/O performance (e.g. Raid 10, Solid State Disks).
This assumes would require the following:
The heap size of the prior level of 600 active users is > 4GB for the heap so I would start with 8GB as a max and 4GB as a minimum and go up from there if you see outofmemory errors in the logs or start to notice slow performance.
Note that the info Branden provided is specifically for the application server - although following his advice to contact an expert might help you with the sizing information for the database server. In general, if you have a DBA on staff I would try to lean on them for help in determining Oracle requirements.
In our organization with 400 users, here are the specs for our Oracle server (which hosts our production JIRA and Confluence):
1TB SAN attached storage (note that our databases are not nearly that large)
...PermissionsStartOnly=true User=www-data Group=www-data ExecStart=/opt/jira/bin/startup.sh ExecStop=/opt/jira/bin/shutdown.sh TimeoutStartSec=120 TimeoutStopSec=600 PrivateTmp=true [Install] WantedBy...
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