Why was the exact number of the People of the Cave not mentioned in the Qur’an, despite Allah knowing the unseen, and does the omission of the number negate the attribute of Allah knowing the unseen?

As for the number of the People of the Cave, some scholars have understood from Allah Almighty's rejection of the first two sayings and His silence regarding the third, that their number was seven, and their eighth was their dog, as Ibn Abbas (may Allah be pleased with him) said. The reason why Allah Almighty left their number ambiguous and turned away from it is to teach proper conduct in such matters: that there is no benefit in delving into them without knowledge, and that it is best to refer the knowledge to Allah Almighty.