Are there always 2 extra days?
All open expeditions may add up to two (2) extra days if bad weather conditions prevent the regular itinerary. That's why the itineraries are expressed as 18+2, 17+2, or 12+2 days.
These extra days are included on the mountain only. If they are not taken on the mountain, they are not included outside the Park.
Private expeditions may have +/- extra days if stated during booking.
See also:
What happens if we don't take the extra days on the mountain?