Answer: About 30% of them died during the voyage
Explanation: The journey was between 6 to 8 weeks, with the Africans tightly packed together to maximise the profits for any given trip. This led to poor hygiene and before long there was dysentery on the ships. Dysentery was the single biggest killer on board slave ships during the middle passage and on average 30% of the human cargo didn't make it to America.
The survivors of the inhumane voyage were sold into servitude, mostly plantations for agriculture. In some colonial states like Virginia, slavery even became a heritable condition based on the mother's status.