Actually, there are 6 different cases (aluminum, space-grey aluminum, stainless steel, space-grey stainless steel, gold and rose gold), effectively doubling the combinations to 1008. And as there are 38 and 42mm sizes, with different (in size) straps for each size, there are 2016 combinations.
Still far from millions, unless we count the change in the color of the time/date font (some faces allow this) as well as the option to rearrange some "modules" in some watch faces...
The 504 was already taking into account the 2 face sizes, so we are still barely cracking a thousand. Here's hoping for custom faces, I can only imagine what some creative devs are capable of.