Naruto had two layers of seals within him. The first was the Eight Trigrams Seal placed by Minato Namikaze and his wife, used to seal the Nine-Tails.
The second seal was the Five Elements Seal that Houka had placed afterward, used to restrain the Nine-Tails' Chakra.
The reason for this was that Naruto had been too young at the time to control the Nine-Tails' Chakra at all, so Houka had used the Five Elements Seal to lock it away.
But now, Naruto had grown up, and his will had become quite firm. It was time to undo the seal and let him gradually come into contact with—perhaps even directly use—the Nine-Tails' Chakra!
That was precisely why Naruto could summon nearly a thousand Shadow Clones at once!
Nearly a thousand Narutos appeared with battle cries, like divine soldiers descending from the heavens. Sakura was both shocked and delighted, realizing for the first time just how dependable Naruto could be!
But Uchiha Sasuke's expression changed drastically as he shouted, "Naruto, you idiot! I don't care whether you came to save me just to show off, but get out of here right now!! Run! This guy is far beyond anything we can fight!"
Unfortunately, Naruto's blood was boiling. How could he care about any of that?
Nearly a thousand Shadow Clones instantly surrounded Orochimaru from every direction and launched into a savage beating!
"It worked!" Naruto cried in delight, but his expression abruptly changed.
One after another, the Shadow Clones were blown apart by Orochimaru, turning into smoke and vanishing on the spot.
"Damn it!"
The instant disappearance of nearly a thousand Shadow Clones made Naruto's head spin, but he quickly recovered. Pressing his index fingers together, he prepared to summon more Shadow Clones!
"Stop, that's enough!"
Uchiha Sasuke suddenly spoke up.