A numeral (saṃkhyā) may be compounded with a word denoting 'one belonging to a family' and the resulting compound is avyayībhāva ,
The word वंश means uninterrupted descent of persons from a common source whether through birth or knowledge. A succession of teachers and pupils, of ancestors and descendants, &c., would constitute a वंश 'family.' One born in or belonging to such a वंश is called a वंश्य or descendant.
As, दिमुनि व्याकरणस्य 'of the Vyakarana,' which has two sages as its principal expounders viz., Panini and Patanjali. So, त्रिमुनिव्याकरणस्य 'having three representatives viz., Pånini, Patanjali and Sakatayana or Katyayana.'
When we desire to express their equality as regards knowledge, then both the words are put in the same case: as द्विमुनि or त्रिमुनि व्याकरणम् .
So also when the relationship is by birth: as, एकविंशति भारद्वाजम् Twenty-one Bharadwajas. These a subsequent compounds are like Dvigu.,