Why did Allah Almighty not tell us the number of the People of the Cave, even though He specified one dog, and despite the importance of Him informing us so that we may protect the believers, given that Allah's not knowing the number would be considered disbelief in Him?

Allah Almighty must be respected, and it is not permissible to question with "Why did Allah not want to inform us?" or to assume that He Almighty does not know, as this is disbelief (kufr). Scholars have known the number of the People of the Cave with documented evidence. If the number is not important and what matters is Allah's protection of the believers, then one should not object to not knowing the number and making false assumptions.