In your account settings on the website, you can cancel your subscription at any time. However, you will lose access to all Tracks Pro features at the next billing cycle.
No, tracks you access as a tracks pro subscriber are available in the Prime app only.
You'll have access to the tracks you unlock for as long as you are an active subscriber. When your subscription cancels, you will lose access to any track you unlocked while subscribed.
As many as you want, as long as you remain a subscriber.