Shows the count of enemies who are (approximately) in range of Fan of Knives. Make sure you are either moving or jumping when you cast Fan, it increases the range noticeably through leeway, and this aura assumes you are either moving or jumping.
It only shows up when 2 enemies are in range, feel free to increase this threshold in the Custom Options.
I also recommend something to show you how many enemies did indeed get hit - https://wago.io/SnvMF0_IA this aura tells you who your Blade Flurry is hitting, and how many targets you successfully hit with Fan of Knives.
Range checks for spells with leeway are unfortunately imprecise with the Blizzard API. This has been correct in my experience but could have some edge cases where it provides false information - feel free to report those in the comments.