how does the currency supply of a country is decided, I searched got that it should be equal to GDP is this true or it's an ideal condition
No, it isn't true. Money that can be printed should be backed by gold. For countries which applying pseudo gold on the money printing system then the supply of currency is depend on the number of gold in that country. The number of gold and paper money that printed should be equal. If the number of paper money is more than the number of gold then it can make inflation rate increase.