The numbers I stated were below:
4.1 million (3.1 million Europeans, 600,000 Slaves, 400,000 mixed and free persons of colour).
Keep in mind that mixed race peoples played an important role as agents of colonisation in various empires, especially the Portuguese empire. The Asian Empire became by and large the work of mixed race individuals and creolized natives. The slave trade in Portuguese Guinea was largely controlled by mixed-race Cape Verdeans. Communities of Portuguese ancestry lasted in Ceylon and Malacca well into the 20th century, long after Portuguese rule had ended. In Timor and the Lesser Sundas they were what kept Portuguese sovereignty alive in those largely neglected islands in the 19th century.
In OTL the region comprising Angola, Mozambique, Zambia, Zimbabwe, Malawi, Namibia, South Africa, Botswana, Leotho and Swaziland had perhaps 4-5 million people by 1700.
Let us not forget that between 1500 and 1850 Africa's population declined. Between 1700 and 1850 by about 23-25 million people (around 16 million of that number was due to the slave trade). The introduction of maize and manioc from the Americas in the 18th century, helped alleviate the population decline somewhat. In this TL we have the usage of slaves in large numbers in Southern Africa itself. This is going to have negative repercussions on African population growth in the region especially due to the constant need to replenish the slaves on the sugarcane plantations.