I have saved 1,500 Dreams and then spent some of them. What can I do with the remaining balance? Do I need to save until I have 1,500 Dreams again?

To start using your Dreams, you need to save until you have 1,500 of them only once. After you have done that, all your old and new Dreams will be active, regardless of the balance. All unused dreams remain on your account and can be spent any time.