Architecturally, how are you storing Git data? If it isn't fundamentally different from bare-repo-on-disk with libgit2 or wrapping the git binary, you're not going to scale any better.
Between the git hosts things move around based on available space, IO pressure and whether it’s been recently accessed or not (spinning disks are far cheaper these days). Theoretically there will be a point where the routing becomes a problem, but probably not until there’s several hundred git hosts (I’ll probably have bandwidth problems before that happens).
I’m not sure what the largest git repo is that github has, but I can theoretically keep repos up to 1TB around (plus a sheer unlimited number of forks for the same). I imagine the API’s get problematic before git does at that point (e.g. shuffling data around is much harder than storing it).