Atlassian tends not to give exact numbers of how many users because there are just too many variables to give a meaningful answer, here. Among other things, it depends on:
There is simply no way to answer this question with so many other variables left open. The best we can do is pick our own data set, hardware, and usage patterns and report the results. That is what the JIRA Data Center Performance and Confluence Data Center Performance pages cover.
The only way for customers to get simple single-valued answers like this is to perform their own testing with the hardware that they intend to use and data and user usage patterns that model their own environment. This fixes the variables to what matters most for them.
Actually, besides the license threshold according to the license you bought for JIRA or Confluence, Atlassian does not limit the number of concurrent users on an application.
That being said, the actual number of concurrent users will depend only on technical aspects of your infrastructure, like your servers' capacity (e.g. processing cores, RAM, I/O speeds...) and/or your network capacity (e.g. link speed).
With Data center, it also depends on the number of nodes. More the nodes, better the performance when there are large number of concurrent users. Maximum users on each node will depend on hardware, as Joao already mentioned!
... within reason, of course. Adding additional nodes would at some point reach diminishing returns because the overhead of keeping the nodes consistent would offset the benefits of distributing the user traffic. We have not tried to push past 4 nodes in our in-house testing, and this is also the largest cluster I'm aware of in actual production at this time.
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