each portal (either parent or child) will have a folder in the Portals folder (and there will always be a _default folder which is used by the host). The number is based on the database sequence, so it keeps incrementing if you add and then delete child or parent portals (often jumps in portalID are because you install to a database you've previously used). Child folders get directories created so that we can parse the address and determine the portalID to use, before redirecting to the root. This is not necessary with parent portals as they're already at the root level.

Cathal