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 (w/External PSC)||10|
|Linked vCenter Servers (w/Embedded PSC)||15|
Note: As of vSphere 6.7, you can have up to 15 Embedded VCSA's within an ELM.
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|