Updated: Aug 17, 2018
Do you have 500+ users? Is your user base highly active and dependent on these tools to never have downtime?
Here's some other clues...
Administration of the applications and hardware has become a full time job for one or more people in your organization.
As you add users, issues, projects and custom fields, your response times and performance isn't keeping pace.
Jira upgrades have caused serious downtime in the past or are expected to in the future.
Deployment of new Atlassian tools has been very taxing on your IT/IS teams.
Your user base is increasing and is expected to be 1000+ in the next few years.
Take a look at the Data Center Architecture below. Admins can deploy the tools on an active-active cluster of servers with multiple nodes. This clustering gives the application scale and resiliency. The clusters grow with the user base and throughput increases. You can deploy On-Premise, or with public IaaS options such as AWS (We do this too!).