In order to manager 10 ESXi host you need one instance of vCenter Server Standard plus licenses for the ESXi hosts (Standard edition, or better). ESXi hosts are license by the number of physical processors. So if you have e.g. 10 ESXi hosts with 2 CPUs each, you need to get 20 ESXi licenses in addition to the vCenter Server license.
ESXi hosts with the free Hypervisor license cannot be managed by vCenter Server.
André