Who was Kamsa in his previous birth?

Kamsa in his previous birth was an Asura called Kalanemi. In Swayambhuva Manvantara, Marichi and his wife Urna had six sons. They once made fun of Brahma saying that he had married his own daughter Saraswati. Brahma cursed them. They had to take birth as the sons of Kalanemi on earth. They again took birth as the six sons of Hiranyakashipu and led a noble life. This pleased Brahma and they asked for a boon from him that they should not be killed by anyone. When Hiranyakashipu came to know about this, he cursed them to go to Pathala and live there. They asked for relief. Hiranyakashipu said that after spending a long time in Pathala, they will take birth as six sons of Vasudeva, and Devaki. Their father from the previous birth Kalanemi would take birth as their uncle Kamsa and he would smash them on the ground and kill them as soon as they were born. This would give them relief from the curse. According to Srimad Bhagavata, the queen of King Ugrasena (king of Mathura) was violated by a Gandharva called Dramila. Kamsa was born to her. She disliked the child and cursed her own son saying that he would be killed by a member of his own family. Narada told Kamsa about this. After that Kamsa developed animosity towards Ugrasena and Krishna.

How many children did Dhritarashtra have?

Dhritarashtra, the Kuru king, had a total of 102 children. He had a hundred sons collectively known as the Kauravas, a daughter named Dusshala, and an additional son called Yuyutsu.