觀無量壽佛經疏 3.64

3.64

三、言有五百億者，正顯其數。一界之上既然，遍滿彼國亦皆如是。應知。

Third, the statement "there are five hundred koṭis" directly reveals their number. Just as this is so upon one boundary, it is entirely like this throughout that realm. One should know [this].