how to reset category ID start number after deleting all the categories?

You can read about this here

Thanks, which table do I have to modify exactly for cobalt 8 ?

in res_categories table I don't see any "auto increment" column or value

do I have to add it?

This is primary key. You cannot create 2 categories with the same primary key. Why do you need 1 again? Why next number is not good?

Why do you need 1 again? Why next number is not good?

well, it's just because this is a new site and I would prefer not to start with "103-category name" in the URLs... but I guess it's not so important... I was just wondering if it was possible to reset that number

Yes but this is complicated. You have to run TRUNCATE comamnd on the table and then insert ROOT category with ID 1. You can find SQL in sql/install.utf8.sql file in cobalt backend.

In fact you can just delete this table and then run create table SQL and insert ROOT category SQL.

done! it was not so complicated... thank you

Then you are simple very clever man :)