Voltage selection problem

The LM7812 for linear power supply is rectified and filtered. How much V is the secondary voltage of the transformer?
Linear power supply LM7812 is rectified and filtered. How much V is the secondary voltage of the transformer? I chose 15V. Will it be small after rectification and filtering?
In addition, for 24V output, is it appropriate to choose 30V for the secondary voltage of the transformer?
Reference here
Thanks in advance.