If pendulum eventually stops due to air friction why pendulum of pendulum clock never stops?
It will stop in a few minutes if left on its own. But pendulum clocks have mechanisms to keep the pendulum moving that need to be manually re-energised occasionally. A common one is to have a weight on a chain. Each swing of the pendulum, the weight drops very slightly, giving just enough energy to keep the pendulum moving. And every day or so, someone has to reset the mechanism by pulling the weight back up to the top again.
And the biggest energy loss of the pendulum is not due to air friction but mechanical friction in the clock mechanism of the hands etc that the pendulum drives around.