One question before more answers: are your characters going to have LoDs? Because if yes, usually you prepare even cheaper materials for far distance, and the most expensive ones for near distance. Than there is other approach which I love to talk about, since we are going to have a presentation from Ryan Brucks about this soon, is the actual use of impostors for very far meshes, which I think you can use even for characters that are not even moving, and then use the regular LoD system when they are moving.
So there are so many options… hard to tell if it is generic, but the best way to understand the impact is the actual use case… you said said about having “many characters onscreen” it helps but a better info would be number of tris per char x number of instances x number of materials per char, not matter at first analysis if it is double sided or not, but might evolve depending on the numbers to actually get them into account.