According to the National Institutes of Health (NIH) and the Institute of Medicine (IOM), the Food and Nutrition Board did not establish a UL for Vitamin B12 because of its low potential for toxicity (see reference).
In Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline, the IOM states that "no adverse effects have been associated with excess vitamin B12 intake from food and supplements in healthy individuals".