Why is Tajū Kage Bushin no Jutsu a secret? In the first episode/chapter of Naruto, Naruto steals the scroll containing secret jutsus and, from it, learns Tajū Kage Bushin no Jutsu, the multiple version of the corporeal alternative to Bushin no Jutsu (which they learn in school). While Kage Bushin no Jutsu seems to take way more chakra and is a lot harder on the user than Bushin no Jutsu, it doesn't seem particularly egregious. Additionally, a decent number of characters seem to use it, if the Wikia article is to be believed. While most characters who use Kage Bushin no Jutsu don't make multiple clones, it doesn't seem like a massive leap from one to the other. So why is it kept secret and sealed away in the Scroll of Seals when other jutsus that are incredibly hard on a person or even potentially fatal (like opening the Eight Gates) aren't?
It’s a forbidden jutsu because of the amount of chakra it requires. Along with that with certain users of the jutsu it could increase their numbers from 1 to 1000 creating a small army.