द्व्यष्टनः संख्यायामबहुव्रीह्यशीत्योः

Operational rule vidhi

The words द्वि and अष्टन् get an आकारादेश in presence of a सङ्ख्याशब्द, provided that there is no बहुव्रीहि समास and the word अशीति is not present.

Word by word (padaccheda)

Target द्वि- अष्टनः ṣaṣṭhī · ekavacanaRight context संख्यायाम् saptamī · ekavacanaRight context अबहुव्रीहि- अशीत्योः saptamī · dvivacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

Full reading

द्वि-अष्टनः संख्यायाम् अबहुव्रीहि-अशीत्योः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

आत् (आ) is substituted for the final of द्वि and अष्टन् when another numeral follows but not in a बहुव्रीहि समास or before आशीतिः।
Thus द्वादश, द्वाविंशतिः, द्वात्रिंशत्, अष्टादश, अष्टाविंशतिः, अष्टात्रिंशत् ॥ Why do we say द्वि and अष्टन् ॥ Observe पञ्चदश ॥ Why do we say 'when followed by a Numeral'? Observed द्वै मातुरः, अष्टमातुरः ॥ Why do we say 'not when the compound is a Bahuvrihi, or the word अशीति follows'? Observe द्वित्राः, त्रिदशाः, द्व्यशीतिः ॥

Vart:- This rule applies upto one hundred. Therefore not here, द्विशतम्, द्विसहस्रम्, अष्टशतम्, अष्टसहस्रम् ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.