This is a feature that is useful for gauging exactly how popular a course may be, over a period of time (how quickly it is bought), and even for ensuring you can track how many people have bought your course (especially when wanting to test with few users).
On your admin-end, click through to the Course tab and choose the desired course. From here, select the Pricing page:
Now you can input how many times you wish for the course to be purchased, based on each individual learner enrolling on the course. Once you have set your limit , for example "7 sales", and the course has been bought by 7 individual learners, it will end its availability, as the limit will have been reached:
Ensure that you click Save for the Access Date box, to confirm the changes. Now you're ready to go!