How to set anumber of things for the user to get from alist

Need help Database
1

Greetings to you ,I needed some help with my audio books . How can I set anumber of books for the user to get .For example if auser pays $1.99 he/she can save 25 audio books which I setup may be by default or dynamic. When (save is a list of files). An example is the tinder superlikes