WOTC-20 is a historic fan-created format that includes all of the sets produced by Wizards of the Coast, from Base through Skyridge. Given access to such a broad and powerful set of synergistic trainer cards, decks are restricted to 20 trainers in order to facilitate interesting choices and diverse gamplay.
For more information about the rules, ban list, and erratas for the format as well as information on where to find games: https://gamesirl.com/wotc20