The affix ḍāc comes after a bahuvrīhi samāsa denoting numeral except when the last term of such samāsa is bahu and gaṇa,
This Bahuvrihi is formed by rule 2.2.25. Thus उपदशाः, उपविंशाः, उपत्रिंशाः, आसप्तदशाः, अदूरदशाः, अधिकदशाः, द्वित्राः, पंचधाः, पंचदशाः ॥
Why do we say 'denoting a numeral'? Observe चित्रगुः, शबलगुः ॥ Why do we say 'not after bahu and gana'? Observe, उपबहवः, उपगणाः ॥ The difference here is in the accent for had डच् been added, it would have made the final of उपगण acute, but it not being so, the first member retains its accent, namely acute on the first.
Vart:- The Tatpurusha compouds of numerals like निस्त्रिंश &c, take also the affix डच् ॥ Thus निर्गतानि त्रिंशतः = निस्त्रिंशानि वर्षाणि देवदत्तस्य 'Devadatta is more than thirty years old' ॥ निर्गतस्त्रिंशतांगुलिभ्य = निस्त्रिंशः खड्गः 'the sword is more than thirty fingers long'.,