Patch reference

Shadow Fiend gains souls from 2 enemies in a 600 radius every 0.5s, prioritizing heroes. Each individual enemy can provide souls once, with creeps giving 1 soul and heroes providing 3. Can collect souls from up to 4/6/8/10 enemies. After the effect is over, Shadow Fiend loses the souls whose owners are still alive, retaining the rest for 8s

The enemy threshold doesn't limit an amount of souls collected, and limits only the amount of enemies affected. So, at the limit of 10, you can collect souls from 5 heroes and 5 creeps gaining 20 souls. 10 Dummy Targets in Hero Demo mode will provide 30 souls