Pidgey evolves into Pidgeotto starting at level 18 in the mainline Pokémon games.

In most traditional games, that’s the first and only evolution trigger for Pidgey itself:

  • Level 18 → Pidgeotto
  • Then Pidgeotto evolves into Pidgeot at level 36.

If you meant Pokémon GO, Pidgey evolves into Pidgeotto by using 12 Pidgey Candy instead of a level, and then Pidgeotto evolves into Pidgeot for 50 Candy.