According to a recent report by Credit Suisse, 97% of all available bitcoins are held by about 4% of addresses. However, the unfortunate answer to your question is that is is quite impossible to tell how much bitcoin is held by exchanges - perhaps precisely because so many exchanges are not regulated. They have no obligation to make this information known to the public, and so any data available is likely off by a big margin.
TradeSNS hope that the majority of netizens comply with the relevant laws and regulations of the network, and prohibit the release of all kinds of sensitive false information;
At the same time TradeSNS will crack down all kinds of illegal dissemination activities and harmful information, building a harmonious space.