Part 1: (V.18) Add the sequence number for each item/product in the sales.order.line model. 

1. From the sales.order model go to Studio.

2. Click on the “Edit list view” button to go to the sale.order.line model 

3. Add a related field to “sequence”.

4. Activate the Developer Mode

5. (Best practice) Change the label and then the technical name. 

Extra tip: When you finish typing the label, type "enter", wait for studio to load and it's coppied to the technical name.

6. Go to the a Sale Order and add a product in the sale lines to see how it works. 

We haven´t finished! The sequence starts with the number 10 and we need it to start from 1) Please go to the next step

7. In order to predefine the number 1 for the sequence as the default. Go to Settings / Technical / Actions / Use-defined defaults

8. Create a new User-defined Default 

• Add the field: Sequence (Sales Order Line) -> You might need to search for it on the "search more" option
• Default value: 1

If you don't find the field "search more" with these values: field: sequence, model: sale.order.line

9. Go to the sale.order model and see how it works. 

Note: You need to save it, in order to have a correct sequence. If you don't save it, you will always have the number 1 in the sequence field. 

Once it's saved you will have the correct sequence. 

Forum post part 2:

(V.18) How to add the sequence to the PDF?

1. From the sale.order model go to Studio and find the reports you want to add the sequence field.

2. Insert a field 

3. Change the name of the field

4. In the rectangle below Type “/” and select “field”

5. Add the native field of Sequence (integer): 

6. Don´t forget to Save it 

7. Print the PDF and the sequence field will be shown.