Chapter 167:
The stage between the Mangekyo Sharingan and the Rinnegan is known as the Eternal Mangekyo Sharingan.
It's achieved by merging two pairs of Mangekyo eyes, typically those of siblings, resulting in a drastic enhancement of spiritual power. This fusion is what allows for the true awakening of the complete Susanoo, the ultimate form of the Susanoo technique.
But Orochimaru, having experienced the Mangekyo, knew this wasn't an absolute requirement.
Why does the Eternal Mangekyo surpass the regular Mangekyo? It's due to the fusion of two Mangekyo pairs, which causes a qualitative leap in spiritual energy. The reason sibling eyes are needed for this is due to the genetic compatibility required for a successful merger without rejection, brothers' genes are typically close enough to avoid the risks. In theory, however, a similar effect could be achieved by father and son, but no one had ever attempted it to confirm.
In short, if one's spiritual energy was strong enough, it was possible to activate the complete Susanoo without the Eternal Mangekyo.
For instance, Uchiha Obito, without possessing the Eternal Mangekyo, was still able to manifest the complete Susanoo after transferring his power to Kakashi. This was possible only because of the immense spiritual power he wielded, having been a Jinchuuriki of the Ten-Tails, which vastly amplified the chakra and spiritual power channeled through his eyes.
Currently, Orochimaru had amassed an immense amount of spiritual power through his experiments, but the limitation was clear: there's a fundamental difference between the regular Sharingan and the Mangekyo. The genetic samples he had collected from ordinary Uchiha bloodlines were only enough to push him to the level of a Mangekyo user, but they didn't contain the kind of spiritual potency required to reach the Eternal Mangekyo.
The fusion of two Mangekyo pairs isn't as simple as doubling power; the synergy produces exponential growth in spiritual strength. So, gathering hundreds of ordinary Sharingan genes would do little to elevate him further, it would barely make a dent.
If he truly wanted to reach the level of a complete Susanoo, he needed genetic material from Uchiha with the Mangekyo itself, someone like Uchiha Madara, Obito, or Itachi.