That depends on what you mean. Let me explain.
When you have bitcoin it is stored in your bitcoin wallet. Each wallet has a public address. (Think of it like an email address) If someone wants to send you bitcoin, they send it to your public address.
Every transaction is posted on the blockchain. In other words, you can see every bitcoin transaction that has ever happened. Because of this, you can see the balance of every bitcoin public address.
So you can trace where bitcoins have been transferred to since they were first mined.
Now that being said, a bitcoin public address doesn’t tell you who the person is, it looks like this
If you want to find out who is sending money to who you need to figure out who owns the wallet. The only way to do this is by finding a place, like an exchange, where the wallet’s public address is connected to the person’s real name. Most exchanges will ask for the person’s name when they use the exchange so at that point you know that that person owns the bitcoins on that exchange and if they transfer it anywhere you can follow exactly where the bitcoins go.
So if your question is, “Can someone find out how many bitcoins I have?” the answer would be; it depends on if you have ever used an exchange or any place else that is connected to your real name. If so, then yes. If not, then probably not.
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