Azure App Service High Density Scaling Tests based on https://docs.microsoft.com/en-us/azure/app-service/manage-scale-per-app. Tested running on PowerShell Core 7 with Az Cmdlets and .NET 3.1 SDK.

CreateHighDensityAppService.ps1

param (
    [string] $ResourceGroup = "rbtesthighdensity",
    [string] $Location = "northeurope",
    [string] $AppPrefix = "rbaathd",
    [int] $PlanScale = 4,
    [int] $AppCount = 8,
    [int] $WorkerScale = 1,
    [bool] $FirstAppLarge = $true
)

$ErrorActionPreference = "Stop"

Write-Host "*** Creating app package"
Remove-Item TestApp -Recurse -Force -ErrorAction SilentlyContinue
mkdir TestApp
Set-Location TestApp
dotnet new web
(Get-Content ".\Startup.cs") -replace """Hello World!""","Environment.MachineName" | Out-File ".\Startup.cs" -Force
dotnet publish -c Release -o Publish
Compress-Archive -Path ".\Publish\*.*" -DestinationPath "..\TestApp.zip" -Force
Set-Location ..
Remove-Item TestApp -Recurse -Force -ErrorAction SilentlyContinue
Write-Host "*** Creating resource group '$ResourceGroup'"
$rg = New-AzResourceGroup -Name $ResourceGroup -Location $Location
Write-Host "*** Creating app service plan '$AppPrefix-plan' with $PlanScale instances"
$plan = New-AzAppServicePlan -ResourceGroupName $rg.ResourceGroupName -Name "$AppPrefix-plan" -Location $Location -Tier Standard -WorkerSize Small -NumberofWorkers $PlanScale -PerSiteScaling $true
1..$AppCount | ForEach-Object {
    Write-Host "*** Creating app service '$AppPrefix-$_'"
    $app = New-AzWebApp -ResourceGroupName $rg.ResourceGroupName -AppServicePlan $plan.Name -Name "$AppPrefix-$_"
    Write-Host "*** Publishing package to app service '$AppPrefix-$_'"
    Publish-AzWebApp -WebApp $app -ArchivePath (Resolve-Path ".\TestApp.zip") -Force | Out-Null
    if ($FirstAppLarge -and $_ -eq 1) {
        $app.SiteConfig.NumberOfWorkers = $PlanScale
    } else {
        $app.SiteConfig.NumberOfWorkers = $WorkerScale
    }
    $app.SiteConfig.AlwaysOn = $true
    $app.SiteConfig.LoadBalancing = "LeastRequests"
    Write-Host "*** Setting app service '$AppPrefix-$_' workers to $($app.SiteConfig.NumberOfWorkers)"
    $app = Set-AzWebApp $app
    Write-Host "*** Pinging app service '$AppPrefix-$_'"
    $check = Invoke-WebRequest -Uri "https://$AppPrefix-$_.azurewebsites.net/"
    $instance = [System.Text.Encoding]::UTF8.GetString($check.Content)
    Write-Host "$AppPrefix-$_ - $($check.StatusCode) - $instance"
}

SetHighDensityScale.ps1

param (
    [string] $ResourceGroup = "rbtesthighdensity",
    [string] $AppPrefix = "rbaathd",
    [bool] $PerSiteScaling = $true,
    [int] $PlanScale = 4,
    [int] $AppCount = 8,
    [int] $WorkerScale = 1,
    [bool] $FirstAppLarge = $true
)

$ErrorActionPreference = "Stop"

Write-Host "*** Setting app service plan '$AppPrefix-plan' to $PlanScale instances"
Set-AzAppServicePlan -ResourceGroupName $ResourceGroup -Name "$AppPrefix-plan" -PerSiteScaling $PerSiteScaling -NumberofWorkers $PlanScale | Out-Null
if ($PerSiteScaling) {
    1..$AppCount | ForEach-Object {
        $app = Get-AzWebApp -ResourceGroupName $ResourceGroup -Name "$AppPrefix-$_"
        if ($FirstAppLarge -and $_ -eq 1) {
            $app.SiteConfig.NumberOfWorkers = $PlanScale
        } else {
            $app.SiteConfig.NumberOfWorkers = $WorkerScale
        }
        Write-Host "*** Setting app service '$AppPrefix-$_' workers to $($app.SiteConfig.NumberOfWorkers)"
        $app = Set-AzWebApp $app
        Write-Host "*** Pinging app service '$AppPrefix-$_'"
        $check = Invoke-WebRequest -Uri "https://$AppPrefix-$_.azurewebsites.net/"
        $instance = [System.Text.Encoding]::UTF8.GetString($check.Content)
        Write-Host "$AppPrefix-$_ - $($check.StatusCode) - $instance"
    }
}

TestHighDensityAppService.ps1

param (
    [string] $AppPrefix = "rbaathd",
    [int] $AppCount = 8,
    [int] $LoadParallel = 32,
    [int] $LoadDelay = 100,
    [int] $RunSeconds = 30
)

$ErrorActionPreference = "Stop"

Write-Host "*** Running web load for $RunSeconds seconds over $AppCount apps on $LoadParallel threads"
$map = @{}
$start = Get-Date
1..$LoadParallel | ForEach-Object -ThrottleLimit $LoadParallel -Parallel {
    $name = $using:AppPrefix
    $appNum = ($_ % $using:AppCount) + 1
    $name += "-$appNum"
    $map = $using:map
    do {
        $check = Invoke-WebRequest -Uri "https://$name.azurewebsites.net/" -DisableKeepAlive -SslProtocol Tls12
        $instance = [System.Text.Encoding]::UTF8.GetString($check.Content)
        if ($map.ContainsKey($instance)) {
            if (-not $map[$instance].Contains($name)) {
                $map[$instance] += $name
            }
        } else {
            $map[$instance] = @($name)
        }
        Start-Sleep -Milliseconds $using:LoadDelay
    } while (((Get-Date) - $using:start).TotalSeconds -lt $using:RunSeconds)
}
$map | Format-Table @{l="Instance";e={$_.Name}},@{l="Apps";e={($_.Value | Sort-Object) -join ', '}} -AutoSize