What happens if you buy a music license for a small audience but the video blows up?
What then?
You have to buy another license accordingly.
Small audience and blow up? Is it for offline video or just Youtube online video? If you just use it for Youtube then you just need standard license even if your video views more than 1 million. CMIIW
What do you mean “video blows up”? Viral video?
As mentioned above, if it’s for online use like YouTube, a regular license is enough. For on demand/streaming (aka Netflix, pay per view, and similar services), broadcast/tv/radio/cinema, print such as dvd, you need to purchase a suitable license for your audience size.