Which blood sugar reading results in a diabetic coma?

Not all blood sugar. A coma does not result as much from the sugar, as the other metabolic changes. With typical type 1, lack of Insulin gives ketoacidosis. Coma is from acidosis, dehydration, etc and the glu may not be extreme (eg 400). A type 2 may get hyperosmolar coma, where glu is very high, but dehydration and electrolyte abnormality is the biggest problem.