The is a very basic unregulated power supply adapter. There are only a transformer, a bridge diode (or 4 rectifier diodes), and an electrolytic capacitor required to build this circuit.
The transformer is chosen according to the desired of the load. For example, if the load need 12 Volts at 1 Ampere current, so there will be a 12 Volts, 1 Ampere transformer should be choosen. But when designing power supplies or most electronic circuits, you should always plan for worst case.
This means, for a load current of 1 amp, a wise choice would be a transformer with a secondary current GPA of 1.5 Amperes or even 2 Amperes. Allows a 50% greater than the value required is a good rule. The primary winding is always adjusted the value of the local power company.