I tried to find a solution to have many people (not a crowd, just many people) inside a scene and being able to render the scene with an average system (GPU 6Gb). I am not really convinced with the results of the billboards for the 3D results, so I tried the lowres scans of real people. Of course, I can't change the poses, I have only modern people and I should not go too close to avoid showing the low-res aspect of the characters. But at least, I am pretty happy with the result...

The lowrez scanned characters are free models from codame and are available on the Skethfab site (https://sketchfab.com/codame). The car model is from nerdadantes.