License keys contain encrypted information about the purchased product edition and are not tied to hardware.

VMware licenses are 25-character strings that contain letters and digits, divided into 5-character blocks with the “-” symbol. For example, you can install vCenter 7.0 to manage hosts running ESXi 6.7 and ESXi 7.0, but you cannot install vCenter Server 6.0 to manage hosts running ESXi 6.5. Older versions of vCenter Server cannot manage newer versions of ESXi. A version of your vCenter Server must at least match or be higher than the highest version of an ESXi host running in your environment that you want to manage by vCenter. VMware vCenter requires a separate vCenter Server license installed on your vCenter Server. VMware vCenter Server allows you to manage ESXi hosts, create data centers that are logical structures to group ESXi hosts, create clusters, migrate virtual machines between hosts, and so on. In this case, I recommend that you install vCenter Server. If you have multiple ESXi hosts, it is better to manage them centrally.

If you use standalone ESXi hosts, buy the needed VMware vSphere license and install it on each ESXi host. There are two main VMware licenses in VMware vSphere: vSphere licenses for ESXi hosts and licenses for vCenter Server. This blog post covers the principles of VMware ESXi licensing and explains how to install VMware licenses. VMware provides different editions of vSphere and vCenter for customers who deploy VMware vSphere to run VMs and build virtual environments. When you decide to start using VMware vSphere to run virtual machines on ESXi hosts in your environment, you need to define which license to buy and understand how to apply the ESXi license. By Michael Bose Licensing ESXi 6.x/7.0.x and vCenter Server 6.x/7.0.x

Esxi 6.5 license