Many people argue about Itachi's stamina, saying that his reserves are small and that he can't last long in a battle. But analyzing the techniques he used in his fight against Sasuke and his conditions at that time, his chakra pool isn't that bad, he actually has a great amount considering the mangafacts below.
- Itachi used genjutsu three times at the beginning of their fight. His genjutsu as an uchiha is known to be of the highest calibur, so it probably consumes some great quantity of chakra.
1st time
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2nd time
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3rd time
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- Then he used a kage bunshin jutsu. Everyone knows that shadow clones divide the chakra among the clones and use it very quickly, so the person loses a large amount of chakra. So after using the Kage Bunshin technique, Itachi had just half of his original chakra. And yet he managed to make impressive moves later.
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- After casting genjutsu three times on Sasuke and making kage bunshin's, Itachi casted Tsukuyomi on Sasuke. It's his strongest genjutsu and he was able to keep going with at least half of his reserves. Tsukuyomi its self is known to be very taxing to the chakra pool. A jutsu that uses as much chakra as this couldn't possibly be used by a ninja with average chakra reserves. Itachi being able to use this with at least half his reserves is pretty considerable.
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- Itachi also used a big Katon against Sasuke. Remember Kakashi impressed by Sasuke using Katon when he was 12 years old, because the person supposedly must have a large amount of stamina to do this jutsu? So, I guess it says a lot...
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- Right after that, he casted Amaterasu twice. And canonically, Amaterasu > Katon. Also both jutsu's being taxing to the chakra pool, Also note Amaterasu uses considerably more chakra then Enton. Enton is essentially ‘unfocused’ Amaterasu being as it doesn’t appear/focus on a single point. Its basically shape manipulation so it can afford to be spammed.
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-Focusing on the 2nd use of Amaterasu. As you can see MS Amaterasu follows the vision of the user. This is important, because in MS, Amaterasu is extremely taxing. Even later on Sasuke's grabs his eye every time he used it with MS and still cannot ’spam’ this ability. But Itachi swept his gaze, and made Amaterasu or the flames chase his 'Line of Sight' long enough that it engulfed an entire portion of the forest surrounding the hideout on fire. It provided the necessary heat to start the Thunder Storm. This means, Itachi used a colossal amount of Chakra to summon so many flames in that duration. That is a testament to his large reserves.
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- As if everything wasn't enough, Itachi was able to form his Susano'o faster than kirin and hold it till the 4th tier long enough to fight and seal off Orochimaru.
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His chakra reserves aren't as low as some people make them seem. He was able to dry Sasuke's reserves and negg Orochimaru while sick, coughing blood and almost dying, so he was obviously previously weakened before his fight against Sasuke, which probably had some impact in his stamina and fighting ability.
And then note Itachi wasn't trying to win the fight but rather make it believable that Sasuke was able to beat him. So some of his skills shown in this are presumably much more impressive. Also note Itachi's jutsu (Susanoo) failure wasn't due to his depletion of chakra but to the severity of his disease. This is also proven because seconds before dying he was able to implant amaterasu again.
In conclusion,
Itachi's reserves are large because he accomplished most of these feats at HALF his reserves. And note every uchiha chakra is way above average.
