Apple care + and how many months are covered after 12 payments

It will have covered the 12 months you've paid for.

Each payment covers the month in which you are paying it.

If you are paying for Applecare + monthly, the term is for the month you are paying for.It will renew automatically. For single payment plans, the initial coverage period is 2 years. Here are the US terms:

https://www.apple.com/legal/sales-support/applecare/applecareplus/docs/applecareplusnaen.html

https://www.apple.com/support/products/iphone/