Naruto used one tail during his first major fight at the Land of Waves, when he battled Haku. That was his earliest clear use of Kurama’s chakra in the series.

Quick note

The “first mission at the mist” is usually referring to the Land of Waves arc, which includes the encounter associated with Haku and the Hidden Mist connection.

Answer

  • Number of tails: 1
  • Context: Naruto tapped into the Nine-Tails’ chakra and manifested the one-tailed form during that early mission.

TL;DR: One tail.