This site lists a total of 600 such coins. Around 1,800 are given in deadcoins.com. But I am afraid that the real number is much higher. More than 10,000 different cryptocurrencies have been created since 2009 and anywhere from 50% to 80% of them are no longer actively traded. That would give a minimum of 5,000 dead coins, and a maximum of approx. 8,000 coins. If we include the coins that are no longer mined or included in any of the exchanges, then this number will rise further.