Just a quick hit that tells you how many volumes Microsoft Data Protection Manager uses. I got 848…

$dpmserver = $env:computername  #localhost or type your dpm server's name here 
$dpmdisks = get-dpmdisk -dpmservername $dpmserver
$volumecount = 0
$dpmdisks | ForEach-Object {
  $volumecount += $_.VolumeCount
}
$volumecount