I've always wondered this question myself, a quick search on the web and I found this from Crypto Coin Charts, there are 125 exchanges.
The top 10 by volume are:
Bitfinex - Based in Hong Kong
Coinone - Based in South Korea
Bitstamp - Based in Luxembourg
Poloniex - Based in United States
Coinbase GDAX - Based in United States
Kraken - Based in United States
Bittrex - Based in United States
Gemini - Based in United States
HitBTC - Based in United Kingdom
BTC-e /Wex - Based in Russia
Don't see yours, find all 125 exchanges here - https://cryptocoincharts.info/markets/info
With crypto, would you buy this or that?
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