You will find the crypto tax listed in one of the tax guides posted on AN, I will save you looking it up, if you bought your crypto on a regulated Thai exchange you pay PIT (up to 35%) on the profit, if you didn't buy it this way then you pay PIT on the entire transaction amount. All the approved exchanges practice KYC and output data to the TRD every year.
If you sell overseas and bring the money here, it's up to you "if and how" you declare it.