Erik,

Erik Colson <eco@ecocode.net> writes:
> hi,
>
> How can I select all mails which are in a certain mail folder or
> subfolders of it ?

You're probably looking for `path:path/to/folder/**` (e.g. `path:Archive/**`).

Documentation:

* man 7 notmuch-search-terms
* https://notmuchmail.org/manpages/notmuch-search-terms-7/

- Steven