How many days do I have if I missed my payment due date for Apple One?

You will need to contact Apple; I just spent some time looking through various Terms and could not find a definitive answer.

Contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide   -

Apple Support schedule phone call     -

Billings and Subscriptions   -

If necessary, use Twitter or Messages.

Apple on Twitter

Messages chat with Apple