When you deploy a vCenter Server Appliance 6.5 or later, Update Manager automatically starts running as a service in the appliance.

The number of hosts and virtual machines that you can upgrade with Update Manager depends on the size of the vSphere environment that the vCenter Server Appliance is suitable for.

For information about the relation between the vSphere environment size and the corresponding system requirements for the vCenter Server Appliance, see "System Requirements for the vCenter Server Appliance and Platform Services Controller Appliance" in the vCenter Server Installation and Setup documentation here.