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易之家呼吁广大网友遵守网络相关法律法规、严禁发布各类敏感不实信息；
同时TradeSNS易之家将严厉打击各类不法传播活动和违法有害信息，构建和谐的网络空间。