Atkins says you start at 20g, then increase gradually until you stop losing weight. OR it did the last time I read the Atkins book. Just below that figure, and you can continue to lose weight. for some people that is a heck of a lot more carbs than 20g a day...
As for low carbing, we are all different, with different carb tolerances, so you just have to experiment until you find the level that works for your body.