If you are going to add multiple hosts to vCenter and license them, it will take a while if you do it manually. Instead you can PowerCLI script it.

Open vSphere PowerCLI console and connect to the vCenter.

Connect-VIServer -Server  $vCenterServerName

Next run the following commands for each host which add them to vCenter and then license them.

Add-VMHost $hostname  -location $cluster -user root -password $root_password  -force:$true
Set-VMHost -VMHost $hostname -LicenseKey $license_key

To repeat this for multiple hosts, I use cat and awk to generate the command for each host. I am sure there are better ways to do this using. For instance, use PowerShell and iterate through the list in a loop. But I am a Linux guy, I find it easy enough to do it this way.

Once I have the series of commands, I paste them into PowerCLI console.

$ cat listofhosts | awk '{print "add-vmhost "$1  " -location CLUSTER_NAME -user root -password PASSWORD  -force:$true"}'
add-vmhost esxi01.example.com -location CLUSTER_NAME -user root -password PASSWORD  -force:$true
add-vmhost esxi02.example.com -location CLUSTER_NAME -user root -password PASSWORD  -force:$true
add-vmhost esxi03.example.com -location CLUSTER_NAME -user root -password PASSWORD  -force:$true
add-vmhost esxi04.example.com -location CLUSTER_NAME -user root -password PASSWORD  -force:$true
add-vmhost esxi05.example.com -location CLUSTER_NAME -user root -password PASSWORD  -force:$true
add-vmhost esxi06.example.com -location CLUSTER_NAME -user root -password PASSWORD  -force:$true

$ cat listofhosts | awk '{print "Set-VMHost -VMHost "$1  " -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX"}'
Set-VMHost -VMHost esxi01.example.com -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Set-VMHost -VMHost esxi02.example.com -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Set-VMHost -VMHost esxi03.example.com -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Set-VMHost -VMHost esxi04.example.com -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Set-VMHost -VMHost esxi05.example.com -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Set-VMHost -VMHost esxi06.example.com -LicenseKey XXXXX-XXXXX-XXXXX-XXXXX-XXXXX