I went with 1 because there is no "less than 1" option.
I count every day on which the party makes wandering monsters checks as an adventuring day, even if no encounter happens or the encounters don't turn into fights.
On three occasions during 19 games played, the players went through a whole dungeon in one go, which I guess would be 5 or 6 fights in one day, but that's been the exception. There's also been a couple of days with 2 or 3 fights.
Though now that the campaign has concluded, I'm not going to run this type ipof campaign with 5th edition again. My unhappiness with 5th edition was the reason I concluded the campaign after the first arc instead of following up with a new arc for the same party.