The recommended daily dosage of vitamin B12 is said to be 2.4 mcg for people over 14, according to the National Institutes of Health Office of Dietary Supplements. However, much of it depends on age, lifestyle, and specific situation.
The recommended daily intake (RDI) for pregnant women is 2.6 mcg. For breastfeeding women it is a bit higher with 2.8 mcg, as per the health body.
Since older people are more susceptible to vitamin B12 deficiency, the National Academy of Medicine recommends that adults over the age of 50 to resort to supplements and fortified foods.