Thank you gentlemen for your input.
One of the footnotes in the paper linked by Frank shows an estimate for Zululand of 200,000 and gives the black population of Natal at about 320,000 in 1879. Apparently the Whites in Natal, mostly British immigrants, were outnumbered 15 to one by the Africans, so a figure approaching 350,000 in total would be about right. I believe Colenso stated the Zulu army totalled 35,000 (approximately 35% of the entire male population!).