Disable YouTube Auto Play:

By default, YouTube will play another video automatically five seconds after you finish one. This can be an annoyance if you show a clip in class and begin discussing it, only to be interrupted by a new video.

To disable, open any video, then:

Click the gear icon at the bottom right of the video player
Toggle the Auto Play button to off

If you are signed in to Chrome or YouTube, this setting will be saved to your account.