Don't they just pay a certain amount per "digital licence"/how many copies they can lend? For instance, my library has an X amount of "copies" of ebooks it can

Don't they just pay a certain amount per "digital licence"/how many copies they can lend? For instance, my library has an X amount of "copies" of ebooks it can lend, I actually never use it because anything that interests me is always "out of stock" and you have to reserve the next available slot.