In a currency exchange rate what is the maximum number of decimal places used?

Nowadays, to support the unlikely exchange rate of XBT/VND, you'll need 10 to 14 decimal places

As of 2017-11-20,

1 XBT = 185,416,429.63 VND
1 VND = 0.00000000539327 XBT

I once worked on an international financial C# based package, which had to support numerous currencies, and we supported 6 decimal places. We had a couple of very experienced designers who told us that would be sufficient.