How many prayers must be redone due to praying in trousers with a tear above the knee?

The thigh is considered 'awrah (private parts) according to the majority of scholars. Neglecting to cover it due to ignorance of the ruling does not invalidate the prayer, according to Shaykh al-Islam Ibn Taymiyyah and those who agree with him. There is no need for you to repeat the prayers if you believed that the thigh is not 'awrah, as this is a valid justification for many scholars. You are permitted not to make up the prayers. However, if you wish to be cautious, then make up whatever prayers would clear your conscience.