How common is it for someone to need a B12 test?

Often. It is used for many indications, often to check for causes of low blood count (anemia) or nutrition malabsorption.
Not a routine test. We screen for B12 deficit when pts have anemia, or had gastric surgery or when they have neuropathies or dementia. Also during malabsorption or malnutrition. It is not a routine screening test.