There are a few possible misunderstandings:
* He loses his arm in the war: During the Fourth Great Ninja War, Naruto uses the power of the Nine-Tailed Fox to create a massive Rasengan that completely obliterates a Ten-Tails attack. This process heavily strains his body and leaves him with a huge amount of chakra exhaustion. However, he doesn't lose his arm, just experiences extreme fatigue and nearly collapses. This happens in episode 330, "Naruto's Decision."
* He gets injured multiple times: Throughout Shippuden, Naruto sustains numerous injuries during battles. He is particularly wounded in his battle with Pain (episode 163-168), where he takes several blows but does not lose an arm. He also suffers from various injuries throughout the war, including being stabbed by Madara (episode 322).
* You might be thinking of other characters: Some characters like Sasuke and Kakashi lose their arm during the series.
To clarify, Naruto never loses his arm in Shippuden. Please let me know if you have any other questions!