In Diablo 1 there was a soft-cap based in the inventory space¹ and Diablo 2 there was a cap based on your level, the maximum gold that you could carry in your hero, and in your chest. (also in world of warcraft)
I'm worried because the gold is shared between my account in diablo 3 (different from diablo 2 where you could split in other heroes).
Best Answer
Nobody outside of Blizzard knows the specific number yet, but the number is at least 100,000,000,000 as that's the max amount you can list an item at in the auction house.
Source: http://us.battle.net/support/en/article/diablo-iii-auction-house-functionality
As with every game, there is a technical limitation, but that depends on how the number is stored. A 32 bit signed integer would put the cap at roughly 2 billion(or 4 billion if unsigned), but since the max bid amount clearly surpasses that, I suspect that they may be storing it as a 64 bit integer, putting the technical cap at 18,446,744,073,709,551,615.
Of course, Blizzard is free to make it lower than that number, and probably have done so.