Hi Sam Sung,
I believe the number of users is somehow taken into account as the overall bitrate of the system. For example, if a system has 64 users with each receiving at 10Gbps, then the overall bitrate is the product 64 X 10e9=640e9 and in the bitrate -distance product, the overall bitrate is considered. It makes more sense to me, but I am not sure about it.