Once you've created your Super Video Audience, you'll be able to see how many people are inside on the "Audiences" page inside your Facebook Ads account.
When that number is lower than expected, there are multiple things that could cause that.
1) It takes a while for Facebook to update the audience
After creating the audience, please give Facebook 30-60 minutes to fully update the audience. Until that time, it might show impartial data.
2) Lower your watch barrier.
Lower the barrier of how long viewers should watch your videos
3) Increase retention
Increase the retention — how many days its been since they've watched your videos.
4) Check how many views you have
In case the above doesn't help, it's possible that you don't have enough views on your videos yet. In that case, it's advised to promote your videos, so more people will watch your videos (and then be included inside your Super Video Audience).
As a general rule of thumb, it's good to have at least 1000 people who have viewed your videos to create.
5) Some of your videos might not be included
At the moment, some of your videos might not be included in the Super Video Audience depending on where you uploaded your videos. You'll find more information about this here. Facebook is working on resolving this.
If you still have issues after trying the above, you can always reach out to your support heroes at firstname.lastname@example.org. We got you :)