Extending/Addition of more VDIs in existing hostpool

Jasmer 

After deploying a hostpool, can we extend or add more VDIs in that pool later on?

I could achieve that in the following way.
Add Session Host: Install WVD Agent and WVD Agent Boot Loader on the target VM.
In addition, Token of HostPool is required at the time of Boot Loader installation.

Reference: https://docs.microsoft.com/en-us/azure/virtual-desktop/create-host-pools-powershell

# Get a Token
$RegInfo = Export-RdsRegistrationInfo -TenantName <tenantname> -HostPoolName <hostpoolname>
$isExpire = $RegInfo.ExpirationTime -lt (Get-Date)
if($isExPire){
Remove-RdsRegistrationInfo -TenantName <tenantname> -HostPoolName <hostpoolname>
$RegInfo = New-RdsRegistrationInfo -TenantName <tenantname> -HostPoolName <hostpoolname> -ExpirationHours <number of hours>
}

Extend Session Host:Change the size in Azure Portal.

In a Personal/Pool based hostpool, is there an auto extension available based on demand?

It is described in the MS document.
https://docs.microsoft.com/en-us/azure/virtual-desktop/set-up-scaling-script

Currently, settings can not be made in Portal, and it seems to incorporate scripts into the task scheduler of session host.
You can, of course, use Azure Automation(Hybrid Runbook Worker) instead of the task scheduler,
There is no big difference between them.