The affixes prāgvateṣṭhañ [[5.1.18]] etc. come after a word denoting a numeral in the sense of 'this is its measure when the word so formed means a Name, a Multitude, a Book or a fixed way and mode of study.,
The phrase तदस्य परिमाणं is understood here. The word in construction to which the affix is to be added should be in the nominative case. When it is a Name, the affix does not change the sense of the radical. Thus पंचकाः = पंचः 'the Five' as पंचका शकुनयः, त्रिकाः 'the Three' = शालङ्कायनाः ॥ The sense of the primitive is changed when it means संघ 'a multitude or flock'; as पंचकः संघः, अष्टकः ॥ So also when it means a सूत्र 'a Book', as, अष्टावध्यायाः परिमाणमस्य सूत्रस्य = अष्टकं पाणिनीयम् 'The Ashtaka of Panini viz, a Book comprising Eight Adhyayas'. So also वैयाघ्रपदीयम्, त्रिकं काशकृत्स्नम् ॥ Is not a collection of Adhyayas the same as संघः ? No, the word संघः is confined to a collection of living beings only.
The word अध्ययन means 'study'. Its numerical measure (संख्या परिमाण) means the particular number of times, or ways in which it should be read. Thus पंचकोऽधीतः 'what is studied five-times or in five modes' सप्तकोऽधीतः so also अष्टकः or नवकः ॥
Vart:- The affix ड comes after the words पंचदश &c, when the word means a Stoma. As, पंचदश मन्त्राः परिमाणमस्य = पंचदशः स्तोमः, सप्तदशः स्तोमः &c. The indicatory ड् causes the elision of final (टि) by 6.4.143, in the cases of words like एकविंशः &c. In the case of पंचदशः the न् is elided by 6.4.141.
Vart:- The affix डिनि comes in the Chhandas after words ending in शत् and शन्, as, पंचदशिनोर्द्धमासाः, त्रिंशिनो मासाः ॥
Vart:- So also after the word विंशति, as विंशिनोऽङ्गिरसः ॥,