I use the repeatable region to show a list of products.
I would like to show the intern ID (of the saved couch data row) to each product.

e.g.:
green car (Item No. 1)
blue car (Item No. 2 )
red car (Item No. 4)
black car (Item No. 3 )

I use this for checking the primary key. I know it starts with 0. How do I get the ID-number in my repeatable region.

[KK]

Could you expand on that bit more please?
The "primary key" part specifically.

The rows in an editable region don't have any fixed id. Also since the rows support getting moved up-down we cannot use their position in the table for anything persistent.

How exactly are you planning to use the 'primary key'?

I need a primary key for my product-id. I work with this number for my checkout procedure. Therefore I need a auto increment id. I thought the article itself has an id I can use.

[KK]

I thought the article itself has an id I can use.

That is right - all cloned pages do have a unique ID.
However you are using repeatable regions and these do not have any ID, I am afraid.

No Prob, is it possible to enter a string that is unique in the admin section? Productss can't have the one product number more than once. Maybe something like 0001, 0002, 0003,...