After the aṅga (stem) aṣṭā (the form assumed by aṣṭan -- aṣṭana ā vibhaktau [[7.2.84]]), auś is substituted for the endings of the Nominative 1st-Case and Accusative -2nd case.,
As अष्टौ तिष्ठन्ति, अष्ठौ पश्य ॥ Why have we taken the form अष्टा and not अष्ट ? Observe अष्ट तिष्ठन्ति, अष्ट पश्य ॥ This peculiar construction of the present sutra (अष्टाभ्यः instead of अष्टनः) indicates, that the आ substitution for the न् of अष्टन् is optional. This sutra is, moreover, an exception by anticipation, to the following sutra, by which the plural Nominative and Accusative endings are elided after the numerals called षष् ॥ The elision of case-endings taught by (II. 4. 7I), is not, however, barred by this rule; the elision taught therein will take place, whether this rule applies or not. Thus अष्टपुत्रः, अष्टभार्यः ॥
The present rule applies even when अष्ट is at the end of a compound, as परमाष्टौ, उत्तमाष्टौ ॥ But in प्रियाष्टानः, औ does not come, as अष्टन् has not assumed the form अष्टा here i.e. it has not lost its न् ॥,