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The number of entry on a D visa is meaningless in your case.
Because, you will receive a Permesso di Soggiorno, which is a common format residence permit, which allows you to enter/exit/transit/visit Schengen however you like (provided you respect the Schengen clock if you are looking to visit other Schengen states of course).
As a national D visa is only used for initial entry and then the residence permit takes the place of it proving your legal stay in the country
The little differing bit is when you apply for it because this can take months for you to receive the card.
Without the physical card, you would usually be required to apply again for a return visa abroad, before being able to return to the country. With the exception during a renewal when it is common rule to allow you to travel with the renewal application receipt and the expired permit
Italy is differing quite a bit in that sense because, contrary to most other Schengen countries, you can leave and re-enter freely while your first application for a Permesso is processed.
You need to provide to the border official the receipt that you receive from the Post Office when you lodge your initial application and must leave/re-enter the Schengen Area through Italy and not other Schengen states while you don't have your permesso
La polizia del Stato states :
The same facilitated procedure is granted to foreigners who have submitted their application for their first residence permits for employment, self-employment, or family reunification, provided that:
- they leave and re-enter Italy through any Italian external border crossing point (cicular letter 11th March 2009);
- they show their passport or other equivalent travel document, along with the entry visa specifying the reasons of their stay (employment, self-employment, or family reunification ) and the receipt issued by Italian Post offices (Poste Italiane S.p.A.);
- they do not transit through other Schengen countries, as this is not allowed.