[Admin]

For the main forum page (index.php), total PMs is $allpm[messages].
For the inbox display page (private.php), you'll need to add it yourself:

  $ignoreusers="";
  if (trim($bbuserinfo['ignorelist'])!="") {
    $ignoreusers='AND fromuserid<>'.implode(' AND fromuserid<>',explode(' ', trim($bbuserinfo['ignorelist'])));
  }
  $allpm=$DB_site->query_first("SELECT COUNT(*) AS messages FROM privatemessage WHERE userid=$bbuserinfo[userid] $ignoreusers");

then use $allpm[messages].

In any file/template, $pmquota is the total PMs allowed.