Q:

Is there a guide for when to cluster GitLab EE, for example when you reach a certain number of users or projects?

Using a private GitLab EE server, what is the best practice to use when scaling up the number of servers in your cluster?  For example: "For every x users or y projects, then you should consider adding another server to the cluster."

A:

GitLab have defined recommendations for the Hardware Requirements for application servers to cater for a range of users here:
https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/install/requirements.md#hardware-requirements
You may want to consider taking a look at the High Availability guidance for clustering, if you plan to exceed those thresholds or if you require redundancy should one of your application servers hosting your GitLab instance go offline:
https://about.gitlab.com/high-availability/
Note, that due to the decentralised nature of git, it is highly conducive to working offline. Your users will only require connectivity to the GitLab instance when syncing branches or managing issues and merge requests via the Web interface.

