In the list of forbidden techniques, multi shadow clone is a silent mafia. It’s one of those techniques that’s been around since ages but still is a forbidden technique. I did some research on this topic and here’s my theory
The shadow clone jutsu was invented by Tobirama Senju, it gives the user to make replicas of themselves.The shadow clone jutsu distributes chakra evenly to every single clone created by the user. This creates lot of problems for the user as the clones can easily be taken care of by a average genin and leaves the user weak. Clones are better used to distract enemies or learn about an enemy’s abilities. If it was a single clone it won’t create problems, as Kakashi is often seen using it with limited Chakra to learn about his opponents abilities, but multiple shadow clones will quickly drain the users chakra which means they’ won’t last long and eventually die. That’s why it’s a forbidden Jutsu.
The only reason Naruto is able to use hundreds of clones is because of the monstrous amount of chakra he gets from Kurama.
This is my theory. Do you guys have something to add to this?