Game Informer had originally stated in their exclusive magazine that the Pokémon Sword and Shield games will have 18 Gyms. However, in their podcast released today, at 25:56, they have since clarified their comment.
Essentially, The Pokémon Company contacted the Game Informer team to say that something was “lost in translation”. The 18 Gym statement appears to be a general statement, rather than specific to the games themselves.
The actual number of Gyms you can challenge is probably not 18. The reference to major and minor leagues appears to be game lore, rather than reflected in the game itself. As it stands, Milo (Grass) and Nessa (Water) are confirmed for both games, while Bea (Fighting) is Sword exclusive, while Allister (Ghost) is Shield exclusive.
The Gym Challenge is a true celebration of Pokémon battling! Participating Trainers strive to collect all eight Gym Badges, and only Pokémon Trainers who have been endorsed by specific people, such as Gym Leaders, can join in.
Edited by HeroLinik and Sheep.