Right now your Movies table just records the number of copies, and your Rent table records the customer's and movie's identities, not which copy they rented. To indicate that a movie was returned, you'd need the customer's id, which could be inconvenient in many cases.
One way to do it is to number or label each physical copy of a movie, and include the copy number in the Rent table.
A more thorough solution is to add another table - let's call it Copies. It would have at least its own identity, as well as the movie's identity, though you could later include other things like physical condition. In the rent table, record the copy's identity rather than the movie's. When a disk is returned, you could find the current rent record for that copy, e.g. by looking for the copyid in conjunction with a false returned bit, or based on the rentdate.