Hey,

First of all if these tenants are completely unknown from each other and you are thinking about do billing per each of them and to isolate them completely i would recommend you to go over different vCenters per tenant even if they share the same authentication method.

I do not know what is for you a "Tenant" in business perspective but think about future expansion, different scopes, evolution, licensing, growth, etc. Having all the vCenters connected on the same SSO domain makes all of them dependent to each other in a certain point of view. In extreme cases where the SSO fails it can impact the other vCenter Server which will be a huge issue for the tenants that should not be impacted.

Think about everytime you need to plan a maintenance windows or do an upgrade, these tasks are always quite time consuming and you are adding the complexity x9 and in some scenarios even the businesses have different times or methods for doing different tasks.