China (Tang dynasty) could go with 500-700 provinces
Tibet, Mongolia, Korea and other smaller realms in the north-east would require some 150-200 provinces.
Japan can probably go with ~100 provinces.
And South-East Asia (notably Khmer and Srivijaya empires) would most likely need around 200 provinces.
Plus, add quite a bit of sea provinces and wastelands.
Also, I assume the map of Africa would expand a bit to the south (unless they decide to tilt the map even further)
In total, this expansion would add more than 1000 provinces. Probably as much as the current map is having. So yeah, total number of provinces would double.
As for any proper representation, I see 3-4 major expansions needed here.
To be honest, and I may sound selfish, I would prefer if a year-worth of expansions is spent on polishing the current map and making it better.
Don't take me wrong, I would still love to see this content.
But I would like to see how good the game is optimized, as well as to see the current map fleshed out a bit more first.