Did Lord Krishna show His universal form to Duryodhana when he tried to captive Lord Krishna in Dhritarashtra's court?
If yes, then how many people saw His Universal form in this episode?
Lord Krishna showed Universal form to all the kings, ministers, rishis and others who were present in court. It is mentioned in the Udyoga Parva of the Mahabharata,
Kesava, that slayer of hostile divisions, endued with great energy, addressed Dhritarashtra's son, Duryodhana, and said, "From delusion, O Suyodhana, thou regardest me to be alone, and it is for this, O thou of little understanding, that thou seekest to make me a captive after vanquishing me by violence. Here, however, are all the Pandavas and all the Vrishnis and Andhakas. Here are all the Adityas, the Rudras, and the Vasus, with all the great Rishis." Saying this Kesava, that slayer of hostile heroes burst out into a loud laughter.
And as the high-souled Sauri laughed, from his body, that resembled a blazing fire, issued myriads of gods, each of lightning effulgence, and not bigger than the thumb. And on his forehead appeared Brahman, and on his breast Rudra. And on his arms appeared the regents of the world, and from his mouth issued Agni, the Adityas, the Sadhyas, the Vasus, the Aswins, the Marutas, with Indra, and the Viswedevas. And myriads of Yakshas, and the Gandharvas, and Rakshasas also, of the same measure and form, issued thence. And from his two arms issued Sankarshana and Dhananjaya. And Arjuna stood on his right, bow in hand, and Rama stood on his left, armed with the plough. And behind him stood Bhima, and Yudhishthira, and the two sons of Madri, and before him were all the Andhakas and the Vrishnis with Pradyumna and other chiefs bearing mighty weapons upraised. And on his diverse arms were seen the conch, the discus, the mace, the bow called Saranga, the plough, the javelin, the Nandaka, and every other weapon, all shining with effulgence, and upraised for striking. And from his eyes and nose and ears and every part of his body, issued fierce sparks of fire mixed with smoke. And from the pores of his body issued sparks of fire like unto the rays of the sun. And beholding that awful form of the high-souled Kesava, all the kings closed their eyes with affrighted hearts, except Drona, and Bhishma, and Vidura, endued with great intelligence, greatly blessed Sanjaya, and the Rishis, possessed of wealth of asceticism, for the divine Janardana gave unto them this divine sight on the occasion. And beholding in the (Kuru) court that highly wonderful sight, celestial drums beat (in the sky) and a floral shower fell (upon him). And the whole Earth trembled (at the time) and the oceans were agitated. And, O bull of the Bharata's race, all the denizens of the earth were filled with great wonder. Then that tiger among men, that chastiser of foes, withdrew that divine and highly wonderful, and extremely varied and auspicious form.