Genjutsu is a very tricky art. It functions by controlling a person's chakra in their brain to cause them to see visions or illusions. It is a delicate art which not everyone has the intelligence and chakra control to do. The Sharingan possesses a normal genjutsu known as Genjutsu: Sharingan. The Genjutsu: Sharingan usually only focuses on one individual, but as the Sharingan user gets stronger, they can affect more people. Itachi and Shisui for example could affect multiple people with their genjutsu. However, Sasuke's eyes eventually evolves to the point he can catch all nine Tailed Beasts at the same time. So really, a Sharingan can put every one of their opponents in genjutsu if it is strong enough. The thing is, the strong the opponent the harder it is to catch them in genjutsu.