how many glasses of milk should I drink a day?

I’d say zero.

What other mammal drinks the milk of another species? What other mammal drinks milk into adulthood?

The way I see it dairy is the most unhealthy thing we consume.

Milk does have calcium but it is also high in protein. Processing excess protein makes our bodies acidic which makes it leach calcium from its bones to neutralize. Drinking milk to prevent osteoporosis is like trying to put out a fire with gasoline. When you look at the countries with the highest rates of osteoporosis and the countries with the highest dairy consumption they are in fact the same. Dairy does not protect us from osteoporosis.

Muscle comes from a good resistance workout routine. Unless you live on nothing but apples and bananas protein is not going to hold you back. The cult of protein came from assumptions made at the beginning of the last century and the pork, beef, dairy industries continue to promote for their own interests. If you eat a healthy varied diet protein will not be a problem even for athletes.