Get-SPOSpecificFolderStats

Get-SPOSpecificFolderStats.ps1

#Parameters
$SiteURL = "https://company.sharepoint.com"
Connect-PnPOnline $SiteURL -UseWebLogin

$FolderSiteRelativeURL = "/Shared Documents/EC/Projects"
$downloadPath = "C:\temp"


Function Get-SPOFolderStats
{
  [cmdletbinding()]
 
    param
    (
        [Parameter(Mandatory=$true,ValueFromPipeline=$true)][Microsoft.SharePoint.Client.Folder]$Folder
    )

Get-PnPProperty -ClientObject $Folder -Property ServerRelativeUrl, Folders | Out-Null
$Web = Get-PnPWeb -Includes ServerRelativeUrl
#$SiteRelativeUrl = $Folder.ServerRelativeUrl -replace "$($web.ServerRelativeUrl)", [string]::Empty
$SiteRelativeUrl = $Folder.ServerRelativeUrl.Substring($Folder.Context.Web.ServerRelativeUrl.Length)
[PSCustomObject] @{
        Folder    = $Folder.Name
        Path      = $Folder.ServerRelativeUrl
        ItemCount = Get-PnPFolderItem -FolderSiteRelativeUrl $SiteRelativeUrl -ItemType File | Measure-Object | Select -ExpandProperty Count
        SubFolderCount = Get-PnPFolderItem -FolderSiteRelativeUrl $SiteRelativeUrl -ItemType Folder | Measure-Object | Select -ExpandProperty Count
    }
     
    #Process Sub-folders
    ForEach($SubFolder in $Folder.Folders)
    {
        Get-SPOFolderStats -Folder $SubFolder
    }
 

 }
$folder = Get-PnpFolder $FolderSiteRelativeURL
$folderStats = Get-SPOFolderStats -Folder $folder
$outputFile = Join-Path $downloadPath "$($folder.name)_stats.csv"
$folderStats | Export-csv $outputFile -NoTypeInformation

This is carved up from https://www.sharepointdiary.com/2019/05/sharepoint-online-get-files-sub-folders-count-in-document-library-using-powershell.html

He did all the heavy lifting.

Update swapped these:

#$SiteRelativeUrl = $Folder.ServerRelativeUrl -replace "$($web.ServerRelativeUrl)", [string]::Empty
$SiteRelativeUrl = $Folder.ServerRelativeUrl.Substring($Folder.Context.Web.ServerRelativeUrl.Length)