This particular question and its variations have been raised quite a bit lately by our field and customers. For me, this was an opportunity to see if we can provide some additional clarification and help explain some of the nuances that may have been causing some of the confusion around the supported maximums for both vCenter Server and the Platform Services Controller (PSC).
In the vSphere 6.5 Configuration Maximum, there are three specific maximums that helps us answer our question on the maximum number of vCenter Servers per vCenter Single Sign-On (SSO) Domain. I will go through each of the maximums and provide some additional context that will help us derive the answer to our question.
The first is the "Linked vCenter Servers" which defines the maximum number of vCenter Servers that can be supported in an Enhanced Linked Mode (ELM) configuration. What is interesting about this particular maximum is that it actually answers the majority of our question. By definition, an ELM consists of a single SSO Domain. This then means that you can only have a maximum of 10 vCenter Servers per SSO Domain.
vCenter Server Maximum
|Linked vCenter Servers||10|
The second is the "Maximum PSCs per vSphere Domain" which defines the maximum number of PSC's that can be part of a single SSO Domain, pretty straight forward. The third is the "Maximum PSCs per site behind a load balancer" which just adds an additional constraint when using a load balancer with your PSCs.
Platform Services Controller Maximum
|Maximum PSCs per vSphere Domain||10|
|Maximum PSCs per site behind a load balancer||4|
After going through the three maximums, we can conclude that the maximum number of supported vCenter Servers per SSO Domain is 10.
Having said that, what does this actually look like when we take into considerations the PSC maximums and the different deployment topologies and configurations that we support? I believe this is where additional confusion arises as this is not something we have historically gone into great detail. What this means is that what is supported or possible is left up for interpretation which is not a good thing, especially when it comes to configuration maximums.
To better help illustrate the possible configurations, I have created several diagrams outlining some the vCenter Servers with an External PSC scenarios.
This first diagram illustrates one method in which you can reach the absolute maximum number of vCenter Servers and PSCs that can be configured within a single SSO Domain. The reason I say one method is that we know the maximum number of vCenter Server per SSO Domain is 10 and the maximum number of PSCs per SSO Domain is 10, so you can have other permutations as long as you do not exceed the maximums listed above. For example, I can have 10xVC + 10xPSC, 1xVC + 10xPSC, 2xVC + 9xPSC, etc.
vCenter Servers w/Load Balanced PSC
If you are using a load balancer with your PSCs, then you can have up to a maximum of 4 PSCs within an SSO Site and up to 10 vCenter Servers all within a single SSO Domain. The diagram below illustrates one method to reach the absolute maximum in terms of the number of vCenter Servers and PSCs. You can also have other permutations as long as you do not exceed the maximums listed above. For example, I can have 2xPSCs being load balanced connected to 2xVCs and have up to 5 of those instances running within a single SSO Domain.
vCenter Server High Availability (VCHA)
If you are taking advantage of the new vCenter Server High Availability (VCHA) feature which was introduced in vSphere 6.5, the maximum is actually exactly the same as the diagram above. There have been a few questions on whether a VCHA setup counts as 1 or two vCenter Servers and the answer is just 1. The reason is that there is only ever one "Active" vCenter Server communicating with the PSC. The diagram below illustrates one method to reach the absolute maximum in terms of the number of vCenter Servers and PSCs.
Hopefully after going through this article, it is now clear on what are the supported maximums for both the vCenter Server and Platform Services Controller within a single SSO Domain. Lastly, the maximums listed in the vSphere Configuration Maximums are officially tested and supported limits based on the available internal resources and customer demand. Exceeding these limits can potentially have a negative impact to your environment as well as from a support standpoint. Always consult the documentation and ensure you are within all supported limits.