I'd probably consider anything around 100g per day a reasonable start for a low carb diet, and going below 40-50g a day to be very low carb. I think Atkins reckoned 20-25 g a day would guarantee to kick start ketosis - this worked for me and I can stay in ketosis while eating between 30-50g carbs a day. I don't choose to try any higher as for the most part this keeps my glucose readings pretty level, and I don't feel I can go much lower for any length of time and still eat a sustainable, varied and (I believe!!) healthy diet.
But how many carbohydrates other people eat will depend on how
they tolerate them, and what targets they or their doctors may have set, and the best way to discover this is by "eating to your meter". It's a matter of no one size fits all, and I think my answer to Alan's question is "However many carbohydrates you need to bring your glucose levels down to an acceptable level while ensuring that your diet remains healthy and sustainable" and for me personally the most important word here is
sustainable...
Robbity