https://www.gamespot.com/articles/nintendo-switch-online-launches-with-20-nes-games-/1100-6461807/

Paying for Nintendo Switch Online does more than allow you to play online. The subscription offers a few exclusive perks as well, including free access to classic NES games. The online service will launch with 20 different NES titles, and nine more will be added before the end of 2018.

Most of Nintendo's largest franchises had their start on the NES, and several of those games will launch with Switch Online. Paying for Nintendo's online subscription nets you games like Mario Bros., The Legend of Zelda, Yoshi, Donkey Kong, and Super Mario Bros. You'll also get some of Nintendo's earliest sports games as well, including Soccer, Tennis, Ice Hockey, Pro Wrestling, and Baseball. The list of 20 games that launch with Switch Online can be found on Nintendo's website, and we've also outlined them below.