It is generally estimated that around twelve million African slaves crossed the Atlantic during the height of the African slave trade. This massive number excludes the millions who were sold into slavery within Africa and those who were sold in Asia. Because the Atlantic slave trade (as part of the triangular trade with Europe and the Americas) saw the highest volume of African slaves, West Africa saw considerable negative effects.
The volume of people sold into slavery impacted African societies by stunting both diplomatic and technological development. The financial payoff was so substantial that millions of able-bodied men and women were hunted down and removed from their communities to be sold. African communities survived without some of their most capable contributors for hundreds of years. As a result, they fell behind as other countries around the world saw agricultural and industrial revolutions. The slave trade also kept rival communities at odds as they worked against each other to profit from the slave industry (most slaves were prisoners of war).
Ultimately, from a long-term perspective, the lack of development and stability over the centuries made it easier for European nations to colonize the vast majority of Africa, hampering African cultural growth and freedoms.