Vitamin B12

Many says that only vegan have problems with vitamin B12, here are some studies showing that eating meat is not a good source of B12:

ars.usda.gov/is/pr/2000/000802.htm

One more link about it:
findarticles.com/cf_0/m0820/ … html?term=

I just wonder how scientists know exactly how much of B12 is needed for a healthy person?
And do all the persons need the same amount of b12 to stay healthy?