They definitely aren't 1 million in gameplay. With a 16 car race in Forza 5 that'd be 16 million polygons roughly, without even considering the track.
Since
apparently even Driveclub's cars have 250,000 average on the more powerful hardware, with fewer cars per race and at half the framerate, there's no way that Forza pushes 1m per car at 60.
If I had to give an estimate for Forza 5, I'd say that a Forzavista model is around 1 million, since it has all of the interior and engine fully detailed and very few polygonal edges are visible (and since it's only rendering one car it can "afford" to be that detailed). I'd guess that a photomode model is around 250,000, and a gameplay car is around 80,000 - 100,000. For some reason, with AI cars using lower LOD models (I'd say around 40,000 maybe), they don't get more detailed in photomode, so you can see all their rough wheel arches and lights, which is weird.