No - if you allow them to create an account on your website, their mandate ID is then saved with their user profile and WPdirectdebit will use this same active mandate for all transactions. Returning customers will not need to re-enter their details each time.
If you don't want to allow customer registrations on your site, then a new mandate will be set up for every subscription or purchase. While this will work, it is simpler for your customers if you do allow them to register. Registration, mandate set-up and subscription set-up can all be handled on one form.