One estimate suggests that around 15-20% of slaves died during the "middle passage," which was the name for the transatlantic journey that took enslaved Africans from Africa to the Americas. These deaths were caused by a variety of factors, including malnutrition, overcrowding, disease, physical abuse, and suicide.
Another estimate puts the death toll at closer to 30%, which would mean that around 3 million African slaves died during the middle passage. This number is based on a study of slave ship logs from the 18th century.
It is important to note that these estimates are just that - estimates. There is no definitive answer to the question of how many slaves died during the middle passage. However, the evidence suggests that the death toll was significant and that the middle passage was a brutal and dehumanizing experience for enslaved Africans.