Is it necessary for a twenty year old to drink a certain amount of milk daily for calcium to safeguard teeth and bones? if yes how much is necessary?

Yes: You don't have to get it all as milk - other forms of dairy such as yogurt ,cheeses, etc are al good sources of calcium. Certain fish are also excellent sources of calcium. Nowadays, many other foods are calcium fortified - for example many orange juices are. You need 3-5 servings of calcium containing foods daily at your age for optimal calcium intake.

3 cups: Purely milk and no other dairy or vitamins around 3 cups a day is what I generally recommend. I recommend vitamin with D & calcium to all teenagers & young adults.

No: There are other sources of calcium besides dairy! Many people have issues with dairy - could be lactose intolerance or even an immune response to either the casein and/or the whey. Dark leafy greens, broccoli, canned salmon with bones, white beans, dried figs, bok choy, blackstrap molasses, kale, black eyes peas, almonds, oranges, turnip greens, sesame seeds, and seaweed all have good calcium.