The regular Sharingan has three different phases, each marked by the number of tomoe. A Sharingan user will have either one, two, or three tomoe. The Sharingan is awakened by emotionally stressful and intense situations, caused by an Uchiha's response to loss of a loved one or the fear of losing a loved one. It's awakening is always triggered by negative emotions such as regret, fear, sorrow, grief, panic, anxiety, or some such mixture of multiple negative emotions at once.
Phase I Sharingan are the weakest, and as far as I can tell, have little impact on one's vision and aren't all that useful in comparison to the Sharingan's later forms. As far as Sharingan go, a Phase I Sharingan is not super effective and not nearly as helpful or powerful as the later phases.
Phase II Sharingan are decent power-wise, but Phase III Sharingan are unquestionably the strongest, and regardless of how good Phase II Sharingan is, Phase III Sharingan has a noticeable upgrade in vision, as seen by how Sasuke notices a marked improvement in his vision and reaction time thanks to his Sharingan upgrade to the third phase.
Oops! This image does not follow our content guidelines. To continue publishing, please remove it or upload a different image.
Three tomoe is a Sharingan's maximum number of tomoe (unless we are talking about the Rinnegan, which can either have 0 tomoe, 6 tomoe, or 9 tomoe, and possibly more but I'm not aware of the extent of how many tomoe a Rinnegan can have since not much is known about the Otsutsuki clan).
Aside from being able to switch between the regular Sharingan and the Mangekyou pattern, a user can't control the current phase of their Sharingan. For example, a three-tomoe Sharingan user will always have a three-tomoe Sharingan; they can't revert to one tomoe or two tomoe (not that any Uchiha would even wantto revert to a less-skilled version of their dojutsu anyway).
Users typically awaken the Sharingan with either one or two tomoe. Hagoromo is the only exception, as the Sage of Six Paths awakened his Sharingan with all three tomoe, but it's not likely that any of Indra's descendants are able to jump right to a three-tomoe Sharingan without first awakening either a Phase I or Phase II Sharingan.
This is my personal theory, but I believe what level a Sharingan is awakened on depends on the situation that triggers the awakening.
The Uchiha who awoke their Sharingan with a single tomoe include Madara, Sarada, and Sasuke. All three of them awakened their Sharingan in response to strong, vivid emotions, such as intense sorrow, loss, pain, fear, horror, or some combination thereof, but no immediate battle action was necessary. In short, the Phase I Sharingan responds primarily to feelings of grief and loss, and is more of an emotional reaction responding to great emotional distress and sorrow. Madara, Sasuke, and Sarada all awoke their Sharingan with a single tomoe because they were greatly affected by grief and/or fear and thus had an intense emotional reaction to their present scenario, but in all their cases, they weren't a situation where they necessarily needed to fight.
· Madara awakened his Sharingan after the shock of Izuna nearly being killed combined with his friendship to Hashirama being discovered and knowing that they would not be able to be friends anymore.
· Sasuke's Sharingan awakened in response to the shock and grief of having his entire clan murdered by his own brother. Arguably his awakening required more of a fighting response, since he attacked Itachi after awakening his Sharingan, but I think that's more the following rage after his grief awoke his Sharingan, since Sasuke wasn't in immediate danger and didn't technically need to defend himself since Itachi had already said he wasn't going to kill Sasuke, and Sasuke had no clanmates left to protect either.
· Sarada awoke her Sharigan due to the sorrow and grief she felt over not having her father around, which became an intense source of heartbreak and regret.
Indra, Obito, and Itachi awakened (and Sasuke reawakened/reactivated) their Sharingan with two tomoe. All of these Sharingan users awakened the Sharingan at a moment when they needed immediate strength and power to fight, being caught in a situation where they and a friend were in immediate danger. Thus, awakening a Sharigan with two tomoe occurs in response to dangerous battle situations when the user needs a power boost to defend himself and whoever he wishes to protect, born from an Uchiha's desire and desperation to protect a friend or in a life-or-death situation.
· Indra needed to protect Asura from being killed by a wild boar.
· Obito need to protect Kakashi from getting killed by enemies during the war.
· Debatably Itachi should have awakened his Sharingan on Phase I rather than Phase II, though I think in his case it could work either way. Itachi awakened his Sharingan after Tenma's death, and it's unclear if Itachi actually tried to attack Tobi, though Itachi would have still felt the need to defend himself from Tobi. Besides, even after Tenma died, a strong wish that he could have protected Tenma undoubtedly still remained. Certainly witnessing the death of a friend would be a strong enough emotion to trigger the Sharingan, and since Tobi was still there and still a threat, Itachi would need to be ready for immediate battle action in order to defend himself, so I think the situation meets the criteria for awakening a Phase II Sharingan.
· The desire to protect himself and Naruto from getting shredded by Haku's needles was what "reawakened" Sasuke's Phase II Sharingan.
One could speculate that the number of tomoe a user gets is based on a user's power, but if that were the case, Madara should have awakened two tomoe at his age, since he was already a trained and experienced fighter when he awakened his Sharingan, yet his Sharingn only had a single tomoe at its awakening. And Obito was known to not be particularly skilled or powerful among his clan at the time he awakened his Sharingan, yet he awakened his Sharingan with two tomoe.
Age could influence how many tomoe a Sharingan user has when they awaken their Sharingan, but again, Madara should have awakened more than one tomoe, so I think how many tomoe a user awakens is dependent on circumstances, not strength, skill, or age.
