If there is a country where cryptocurrency is legal how much is the tax that we have to pay to the government?
every country has different rules and tax rates. some jurisdictions have no capital gains tax at all. for instance, residents of puerto rico legally pay zero capital gains tax. (although if you move there and then realize past gains, there is a 5% tax during your first 10 years of residency)
Also, how can the government track our transactions?
the easiest way is through regulated channels like exchanges. when you complete KYC with an exchange, they have eyes on the crypto you bring in and out of the exchange. governments also engage with blockchain analysis companies who work to identify your activity even when they aren't directly related to your use of exchanges and other KYC services.