Well, most people can easily enter ketosis with 20-30 grams of net carbs per day. You can increase this to 50 g/day if you're very active and even more if you're an athlete.
The exact amount of carbs a person can handle and stay in ketosis depends on their activity levels and body composition. Athletes can and should eat more carbs than the average ketoer. Most do this before and after workouts through carb cycling.