The size of your cable doesn't necessarily have to be the actual current
you draw. It might be over dimensioned. It indicates only the maximum power you can safely transport through that cable.
You need to know the actual maximum power consumption
of your onboard systems.
Unless you have an airconditioner (might be around 10 amps at 240V or 20 amps at 115 V) or an electric
heater (might also be 10 amps at 240V) or electric
cooker (might be up to 30 amps at 240V, double for 115V), you might never be drawing 50 amps from the shore supply. You need to add al possible appliances
to come to a safe total, so include appliances
connected to an outlet, that is connected to shore power.
On our 36 ft sailboat we have a 16 amp, 240V cable, but we never use more than 10 amps. But we can safely plug into both 30 and 50 amp receptacles.