Oricon's site is a pain in the ass. They only have a short overview
how they count and ask everybody, who wants to know more, to write an e-mail. Sure.
So I've searched the Japanese net and found the info on Wikipedia:
https://ja.wikipedia.org/wiki/%E3%82...83%90%E3%83%A0
So according to this, an album for Oricon is an album, when it's 5 original songs (not tracks) or more. It mentions Ayu's A having 14 tracks, but only 4 original songs, which is why it was counted as a single.
So looks like it only is about the number of original songs. The UK chart e.g. counts singles as albums, when they have more than 4 tracks and/or the length is exceeding 20 minutes.