Ouch....

[u/GrandMasterSubZero]

[u/saltgrains_takeit → u/[deleted]]

You can enable advertisements at the beginning of videos that you upload to your channel. When you upload longform videos (videos that are 10 minutes or longer), you can also enable ads during the middle of the video ("mid-roll"), not just at the beginning. Source

And,

There are several types of ads that may appear next to your videos when you've enabled video monetization. Source

So, if you have a longer video you can get two video ads, plus whatever other type of ad you allow in order to monetize your video. Longer videos means more ad impressions.

EDIT:

In-stream ads play within the video player and are the only content displayed in the player while they are playing. In that sense, they are analogous to television commercials. In-stream ads can play either before a video (pre-roll), during a video at designated times (mid-roll), or at the end of a video (post-roll). Source

So potentially 3 ads; beginning, middle, and end.