The Nominative 1st-Case and Accusative -2nd case plural endings are elided after the numerals calls ṣaṣ -- ṣṇāntā ṣaṭ [[1.1.24]].,
As षट् तिष्ठन्ति, षट् पश्य, पञ्च, सप्त, नव, दश ॥ The rule applies even to compounds ending with 'shash' words : as परमषट्, उत्तमषट्, but the 'shash' should be the principal; if it is only a secondary member of the compound, the rule will not apply: as प्रियषषः, प्रियपञ्चानः ॥ For the elision of न् see 8.2.7.,