俱舍論疏 28.192

28.192

論「無所有處至上地唯淨」，述無所有無間生七。非想唯一，無無漏故。自及下二各淨、無漏二，復有六故，所以成七。

The passage of the śāstra from 'The Sphere of Nothingness...' down to '...the higher stage is solely pure' explains that Nothingness gives rise to seven without interval. The Summit of Existence is only one, because it lacks the untainted. Its own and the two lower ones have each the pure and the untainted, which further make six, and therefore they make seven.