The augment n is added before the Genitive 6th-Case plural ending ām after the numerals called ṣaṣ and after catur,
As षण्णाम्, पञ्चानाम्, 6.4.7, सप्तानाम्, नवानाम्, दशानाम्, चतुर्णाम् ॥ A numeral ending in र् is not 'shash', hence the specific mention of चतुर ॥
This rule applies to compounds ending with these numerals, when these latter are the principal member, as परमषण्णाम्, परमपंचानाम्, परमचतुर्णाम्, but, प्रियषषाम्, प्रियपञ्चाम्, प्रियचतुराम् where the Numerals are secondary (upasarjana).,