When Super Smash Bros. 4 launched, Nintendo released an adapter to support four up to four GameCube controllers on Wii U. The Nintendo Switch received the same treatment when Super Smash Bros. Ultimate launched late last year. The official Nintendo adapter can be, sadly, hard to find, but there are numerous options that get the job done well.
https://www.amazon.com/nintendo-switch-controller-adapter/s?k=nintendo+switch+controller+adapter

