You explained it clear enough. On the playlist on YouTube, the video plays sequencially. In the video addon, it is not possible. It only plays one video.
Please take a raw HTML addon and paste the below code in the HTML Code section. It will show the playlist and the videos will be played in sequence. You will not need 21 video addon then. Only 1 addon and the videos will be played one after another.
<iframe width="560" height="315" src="/www.youtube.com/embed/videoseries?si=T63L54QZnd8ZUMMw&list=PLIDJz4oGAW4PR2SIcky0_DBFJipmabEog" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>