A common rule of thumb is for every Ampere, add 1000 uF capacitor value. It is not a calculated answer but it may be sufficient to show that you know a few design shortcuts.
To make a calculation, you want to find a capacitor value which is high enough so it drives the load during idle gaps.
The RC time constant is the time required for voltage to fall 63 percent. Say your spec is for 5% ripple (voltage drops 5 percent during idle gaps). If mains is 50 Hz, then 1/50 second is idle time in a half-wave power supply. Derive the time constant. Approximately we have voltage falling 63 percent in 12/50 sec. Approximate is good enough.
Divide 12/50 by load resistance and the result is your capacitor value.
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Namely the capacitor value that yields 5% ripple with that load resistance, in a half-wave power supply at 50Hz.