The African Slave Trade Eastwards
The best estimate of the number of Africans taken in slavery to the Middle East and the Indian sub-continent is debated.
The trans-Saharan, Indian Ocean, and East African slave trades transported millions of Africans to the Middle East, North Africa, and the Indian subcontinent over more than a millennium.
It is difficult to estimate the number through lack of records, but historians estimate between eight and fourteen million through the trans-Saharan and Indian Ocean slave trades from the 7th century until the early 20th century.
Estimates suggest up to five million Africans were taken to the Indian subcontinent, mostly between the 9th and 19th centuries as soldiers, servants, and concubines.
So somewhere from ten to twenty million people were transported on an eastward route out of East Africa. That compares to an estimated twelve and a half million people who were transported to the New World.
The sources are Paul E. Lovejoy's Transformations in Slavery: A History of Slavery in Africa, published by Cambridge University Press, and Richard B. Allen's European Slave Trading in the Indian Ocean, 1500–1850 published by Ohio University Press.