Creating custom form at registration process for unknown number of nodes I have a project where I want to add one OR more "nodes" or content types at registration time. For example, I enter my name and email adress, and then I can add one or more trip (example) I made, and then, later, the content of the site itself will be modified by this information. For example, the main menu could display a tab for each trip (trip1, trip2, etc.).

Any idea where I should start?
You can try with 

field_attach_form to attach the 'trip' content type's fields to user form. (with an alter can become multivalue)

on submit you can use these values to populate the trip entity with entity_metadata_wrapper and create a new node

you can build N menu_items than are the trip of the user by a custom menu