As of July 24, 2016, there were 5.6 million Ethereum addresses with a balance of Ether.1 Each address is associated with a unique private key, which is used to sign transactions.
2 Thus, there are at least 5.6 million Ethereum private keys in existence.
It’s impossible to know the exact number of Ethereum private keys because it’s not possible to know how many addresses have been generated but never used. It’s also possible that some people have generated multiple addresses and/or private keys.
Ethereum addresses are generated randomly, and it’s unlikely that anyone will ever generate the same address twice.3 However, it’s still possible for two people to generate the same private key.
This is known as a “collision,” and it’s incredibly unlikely to happen.4.
There are a few reasons why you might want to generate a new Ethereum address:5
You want to keep your transaction history private (e.g., if you’re sending or receiving Ether for illegal purposes).
You’re worried that your private key might be compromised (e.g., if you lost your wallet or had it stolen).
You want to use a different address for each transaction (to avoid being linked to any one particular address).
If you’re generating a new address for privacy reasons, it’s important to use a new private key as well. Otherwise, your old transaction history will be associated with your new address.6
It’s also important to backup your new wallet and keep your new private key safe and secure.7 If you lose access to your wallet or forget your private key, you’ll lose all of the Ether in that wallet.
8 There are no customer service representatives who can help you recover lost Ether; it’s up to you to take precautions to protect your wallet and private key.
Conclusion: As of July 2016, there were at least 5.
However, the actual number is likely much higher due to the difficulty of knowing how many addresses have been generated but never used. It’s important to take steps to protect your own private key(s), as losing access to them will result in the loss of any Ether associated with those keys.