Gets the file and folder count for the provided list folder and, optionally, all sub folders.

Get-FolderChildCount.ps1

#requies -modules "PnP.PowerShell"

function Get-FolderChildCount
{
    [CmdletBinding()]
    param
    (
        [Parameter(Mandatory=$true)]
        [Microsoft.SharePoint.Client.List]
        $List,

        [Parameter(Mandatory=$true)]
        [Microsoft.SharePoint.Client.Folder]
        $Folder,

        [Parameter(Mandatory=$false)]
        [switch]
        $Recurse,

        [Parameter(Mandatory=$false)]
        [switch]
        $Detailed
    )

    begin
    {
        $overrideParameters = New-Object Microsoft.SharePoint.Client.RenderListDataOverrideParameters
        $scope    = ""
        $nextHref = ""
        $counter  = 0

        if( $Recurse.IsPresent )
        {
            $scope = "Scope='RecursiveAll'"
        }
    }
    process
    {
        $results = while($true)
        {
            $counter++

            $renderListDataParameters = New-Object Microsoft.SharePoint.Client.RenderListDataParameters
            $renderListDataParameters.ViewXml = "<View $scope><ViewFields><FieldRef Name='FSObjType'/><FieldRef Name='FileRef'/><FieldRef Name='FileLeafRef'/></ViewFields><RowLimit Paged='TRUE'>5000</RowLimit></View>"
            $renderListDataParameters.Paging  = $nextHref
            $renderListDataParameters.FolderServerRelativeUrl = $Folder.ServerRelativeUrl

            $clientResult = $list.RenderListDataAsStream($renderListDataParameters, $overrideParameters)

            Invoke-PnPQuery

            try
            {
                $streamReader = New-Object System.IO.StreamReader( $clientResult.Value, ([System.Text.Encoding]::UTF8) )
                $text  = $streamReader.ReadToEnd()
                $items = $text | ConvertFrom-Json

                $items.Row | Select-Object `
                        FSObjType, 
                        @{ Name="Path"; E={ $_.FileRef.Substring(0, $_.FileRef.LastIndexOf('/')) }}, 
                        @{ Name="Name"; E={ $_.FileLeafRef }}

                if( $items.NextHref )
                {
                    $nextHref = $items.NextHref.Substring(1)
                }
                else
                {
                    break
                }
            }
            finally
            {
                if( $streamReader ) { $streamReader.Dispose() }
            }
        }

        if( $Detailed.IsPresent )
        {
            return $results
        }
        else
        {
            [PSCustomObject] @{
                List                    = $list.Title
                FolderServerRelativeUrl = $Folder.ServerRelativeUrl
                FileCount               = ($results | Where-Object -Property "FSObjType" -eq 0 | Measure-Object).Count
                FolderCount             = ($results | Where-Object -Property "FSObjType" -eq 1 | Measure-Object).Count
                Total                   = $results.Count
            }
        }
    }
    end
    {
    }
}


Connect-PnPOnline `
            -Url         "https://$env:O365_TENANT.sharepoint.com/sites/teamsite" `
            -ClientId    $env:O365_CLIENTID `
            -Thumbprint  $env:O365_THUMBPRINT `
            -Tenant      $env:O365_TENANTID `
            -ErrorAction Stop

$list = Get-PnpList -Identity "large list" -ThrowExceptionIfListNotFound -ErrorAction Stop

$folder = Get-PnPFolder -Url "$($list.RootFolder.ServerRelativeUrl)/5001 Items" -ErrorAction Stop

Get-FolderChildCount -List $List -Folder $folder -Recurse