The answer, my friend, is blowin' in the wind.
Prior to the above quote, the unnamed narrator asks listeners several questions (like "How many roads must a man walk down before you call him a man?"). The narrator's response to them: "[an] answer" which is "blowin' in the wind." In other words, you could tell everyone the answer to any kind of question, but no one would listen as they don't want to hear the answer - even if it is readily apparent.
Yes, and how many deaths will it take 'til he knows that too many people have died?
The narrator of the song questions how many deaths it will take before people get fed up and say that the war should end and "enough is enough." How many deaths, the narrator questions, will it take for the death and destruction to - ironically - stop?
Yes, 'n' how many years can some people exist before they're allowed to be free?
This references the enslavement and systematic enslavement and oppression of African American people primarily in the United States throughout the 1800s and 1900s. The narrator questions how long and why oppression and slavery could go on. The narrator also questions why it took so long for African Americans to be free (even though they are human beings worthy of dignity and respect).
