What is the ampere rating of a 12 volt battery?

You first need to understand the Rating on a Battery

Voltage rating:- It is the terminal voltage provided by the battery.This Terminal Voltage Decreases as the battery Discharges.

Capacity of the Battery:- It is measured in Ah/mAh. The battery has limited number of electron it can puss through the circuit.The capacity of battery is measure by discharging at a constant rate until it reaches Terminal Voltage.
1Ah means battery can supply 1 amp current for 1 hr or 2 amp current for 1/2 hr till it reaches terminal voltage.

Some Manufactures also Specify the Discharge Rate like 300Ah(10 hr rate) means 30 amp current for 10 hrs till it reaches its terminal voltage.(Normally it should be 300 amp supply for 1 hr).

As a battery discharges, not only does it burns its internal store of energy, but its internal resistance also increases, and its open-circuit cell voltage decreases. The most deceptive change that a discharging battery exhibits is increased resistance.

SO coming to your Question..

A 12 volt Battery current supply depend on its load , the Ah/mAh rating written is in reference to that particular current drawn from it.Also as the Battery Discharges it terminal voltage measured across load Decreases.

This is a typical 12 volt Battery generally used in Converters/UPS .The Ratings are 12V, 50 AH i.e the battery supply 12 volt initially , 50 amp currrent for 1 hr. Remember the voltage across load decreases with time i.e 12v,11v……5v……..upto Terminal Voltage.