I can't seem to find it exactly; But I'll paraphrase what he put down.
Essentially, Genjutsu is still fairly effective against an opponent who can make a mass amount of clones. The clones and the original are clearly linked, such as when Naruto was influenced by the Kyuubi's chakra and the clone's could feel the
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from the chakra despite not being exposed to it themselves. This is important because it proves that there is a chakra signal being sent from the original to the clones. When the chakra is disrupted, the clone's will fade away, this happened to the Mokuton clones that Hashirama had out when he was stabbed with Madara's Outer Path rods. His chakra was disrupted and his clone's couldn't function anymore. Even if Itachi catches a clone in a Genjutsu;
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and
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like what happened to Kakashi's. So, basically Clone's offer little help against Genjutsu considering if the original is hit, they'll be useless.
That's somewhere around what AkatsukiChurch had said.