I summarize my understanding based upon your answer :
Blockchain.info (where I have an account) stores my encrypted wallet and that address has some coins associated (the one I purchased) so when I am hacked hackers can get access to this wallet and in turn my private keys. So to be safe I should probably use cold storage.
But I read on this forum that one of the guys wallet was hacked but users were asking if he still has private keys (not known to hacker) how does that help, because if someone get access to my blockchain account then that means he got everything nope ?
Yes, once someone has the private keys they are able to spend the coins, that's the whole point of bitcoin. Bitcoin doesn't care about the rightfull owner, it only care about the one who knows the private key. If you keep the key secret, you must be the owner.
It does not really make sense what you say (you should change the subject )
The private key does not "contain" coins, it points to an address at the blockchain were an amount of coins is stored.
You can copy the private key thousand times, but they all point to the same public address which is stored at the blockchain (which every node has a copy of)
Every address is very unique and rare.
If there were no transactions made from that address in the past, nobody knows the address.
The address becomes known to the rest of the world when there is a transaction made, then it's a part of the blockchain.
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They know who you are by giving you a unique address. Every order gets a different address. They know you paid when the see the transaction sending bitcoins to that address.
Yes, that is true. They can also reuse the address only for you or generate a new address for every transaction.
Addresses are free to create, so why not.