After you install ESXi, it has a 60-day evaluation period during which you can explore the full set of vSphere features provided with a vSphere Enterprise Plus license. You must assign the host an appropriate license before the evaluation period expires.
ESXi hosts are licensed with vSphere licenses that have per-CPU capacity. To license hosts correctly, you must assign them a vSphere license that has enough CPU capacity to cover all CPUs in the hosts. The license must support all features that the hosts are using. For example, if the hosts are connected to a vSphere Distributed Switch, you must assign a license that has the vSphere Distributed Switch feature.
You can use one of following methods to license ESXi hosts:
License multiple hosts at a time by using the license management function in the vSphere Web Client. The hosts must be connected to a vCenter Server system. For more information, see vCenter Server and Host Management.
Set up bulk licensing by using PowerCLI commands. Bulk licensing works for all ESXi hosts, but is especially useful for hosts provisioned with Auto Deploy. See Set Up Bulk Licensing
License individual ESXi hosts through a direct connection with the vSphere Client. For more information, see vSphere Administration with the vSphere Client.