Why does Satan whisper to the Children of Adam despite his knowledge of the fixed number of people destined for Hell (999)?

It is possible that the questioner is referring to the Hadith which states that the number of people of Hellfire from every thousand is nine hundred and ninety-nine. We cannot assert that Iblis knows the number of those who will enter Hellfire, as this is part of the unseen which only Allah knows. So, perhaps Iblis knows it, or perhaps Allah has withheld it from him. Allah Almighty has linked causes with their effects, so He created Paradise and Hellfire and predestined the causes of guidance and misguidance. Among the causes of human misguidance are the whispers of Satan and his beautification of falsehood. Iblis strived to mislead the Children of Adam and make them deserving of entering Hellfire, and all of this was decreed by Allah. Therefore, stating that the people of Hellfire will enter it whether Iblis whispered to them or not is incorrect, because Allah decreed their entry through causes that necessitate it, including Iblis's misleading them and their following him.