Since you have 18 teams, you cannot play a standard bracket where everyone plays in Round 1. You have two options here, but the standard "Championship" style is the most recommended.
Challonge, Toornament, or Battlefy can auto-generate an 18-team double elim bracket and handle the logic for you.
In double elimination, total matches = (2 × number of teams) – 1 (if no second final) or (2 × number of teams) (if second final needed). For 18 teams, that’s 35 or 36 matches.