What are your Tips for rendering a crowd of Thousands?

Hi All,

I am investigating how to render a crowd/audience for a sports/music style arena and since it is for VR I really need to get this as optimized as I possibly can.
Does anyone have any tips for how this should be approached?
Are there specific ways of instancing Actors/Animations/Meshes that means that I can cram more of them in there?

Cheers