What is the minimum amount of sleep needed to have good brain functioning?

Depends on age. For you age probably 7 1/2 to 9 1/2 hours of good sleep are required for optimal brain function.
6-8 hours. The individual sleep time varies much between each person. The average time is around 6-8 h. I general whatever the time is that refreshes you in the morning. Too much or too little sleep needs your attention.