That depends on your unique physiology and goals. But to keep things simple: 5-10 % of your daily calories should come from carbs on a keto diet if you're an average dieter.
If, however, you’re following keto as a way to fight cancer, you'll have to eat fewer carbs than the average ketoer . On the other hand, if you do high-intensity workouts, then you'll probably need more carbs than someone who's only moderately active.
But no matter how low your carb intake, keep in mind that many food that's important for your health contains carbs (fruits and vegetables). Excluding these completely from your diet is not a good idea since you'll be missing out on important nutrients and antioxidants.