Rather depends on the "Natives" in question!
If rifle armed/Regulars (Boers, Egptians, Afghans/Pathans) 2:1 in favor of the locals is the general rule. Armies with minimal firearms (at least for game purposes) such as Zulu's and Mahdist Ansar, 3:1 is their general rule.
It's easy to over think the entire matter, though. The Ansar could be supported by units of rifle armed Jihadyia which in the opening years were reasonably well trained and experienced (being largely "converted" Egyptian/Sudanese soldiers), but in game terms maybe only one Ansar unit in five might be so designated. The various Beja Tribes of the Eastern Sudan generally had no firearms to speak of.
In practical fact, perhaps half of the Zulu's of the war of 1879 carried some kind of firearm, but could never be thought of as "riflemen" for purposes of play balance.
Boers were obviously a very different kettle of fish, but the idea of their regularly outnumbering the Brits--much less at 2:1--is ridiculous.
The Egyptians were a solidly Regular Army cursed with an abysmal Officer Corps and corrupt government. They COULD be allowed to outnumber the Tommy's 2:1, but shouldn't have to.
Finally, of the NWF warriors fully half had fairly serious firearms, of which maybe five in ten could be presumed to be armed with Jezails, which the rules allow slightly greater range than the Redcoats. For the Second Afghan War, there could be significant numbers of units of "Regulars," entirely rifle armed, but whose leadership/staying power was generally limited. Where such Afghan Regular units participate in a game, the 2:1 figure could once again be an overstatement.
Of course, such generalities don't address the myriad other powers the Brit's faced in the era of the Snider & Martini-Henry (the period the rules best represent), such as Maori's, Abyssinians, Chinese, and Ashanti. Information on these armies appears in the campaign game rules, "The Sun Never Sets," though basic game information tables for these are available for the asking from
TVAG
