In the KB "Installing vCenter Server best practices" they mention that SSO and Inventory Services may be installed on the same server as the vCenter itself or on separate servers, but the only guidelines to when separate servers would be required is this:
"For small vSphere deployments, vCenter Server 5.1 provides a vCenter Server Simple Install option that installs vCenter Single Sign On, Inventory Service, and vCenter Server on the same host or virtual machine.
Alternatively, to customize the location and setup of each component, you can install the components separately by selecting the individual installation options, in this order: vCenter Single Sign On, Inventory Service, and vCenter Server. Each component can be installed in a different host or virtual machine."
So when do we cross the line going from a small deployment to not-so-small?
Yeah I had the same question. It should be fine to have it all on one box. Probably with 50 or more hosts and thousands of VMs better to have it separate?
But SSO is a small service not sure if they need to be separate.
I believe VMware defines "small" environment as 100 or less hosts, or 1000 virtual machines. That's what the vCenter docs seem to say, anyway.
As I have only 8 hosts and 125 VMs, I qualify as "small".