There are 2 dark portals - the one that the Lord Master opens at the end of the book, and the cursed mirrors that Mac sees inside 1247 LaRuhe St. The mirrors served as ancient portals between the realms until they were cursed. It's not clear how many Humans could see either one, but my guess is that certain Sidhe-seers can see them, if that is the flavor of their ability to 'see,' and that it is clear that Mac can see both of them.
