The homework you really have to do yourself-count the nr of travalDAYS you will need (on trains-all trains you board on 1 day count-so it differs if you plan to use overnight or not). It will also depend on if you intend to make stops en-route, like f.e. in Göteborg or so. Its likely 7 or 10-dep. on ferry yes/no.

You have to pay a few € extra for all REServ you may need to make-the highest probably if you stick to the superexpensive Thalys out of Par. IF you plan smartly, you can avoid a lot of that-but now its far too vague to tell all.

Merci pour votre retour. Est-ce qu'il existe un service d'assistance qui peut nous aider si j'ai un projet de voyage bien défini avec des villes et des dates ?

Interrail does not help you with planning; that's your job. Use good travel planners such as DB for international journeys, sj.se for Sweden and entur.no for Norway. Do NOT use the rail planner app to plan: it is not reliable enough for that purpose because it doesn't get enough updates.

Si j'achète un pass interrail 7 jours 1 mois, je pourrais faire 7 trajets sur 1 mois quelle que soit la compagnie : Db, entur, thalys, ... ?

Est ce qu'il y a des offres pour les ferry ?

On any travel day, you can use as many trains of the participating companies as you like.

There are some ferries that offer a discount, see this overview. See the separate ferry pages for how to book. If you only get a discount, then you don't need to use a travel day.

Si sur une même journée j'utilise 3 trains de 2 compagnies différentes Thalys et DB, ça compte comme 1 voyage ?

Oui, all trains that you board between 00.00 and 23.59 CET are one travel day.