They would resupply mid 'battle', much like Titans do, or other armoured vehicles. Depending on distance they would resupply at arming stations and ammo dumps or have it brought to them. Remember they're not deployed on their lonesome, there's a whole mechanicus and feudal bondsman support network behind them.
Each Paladin would carry enough munitions for an engagement and then return to rearm. Realise that a battle is not continuous fighting everywhere at once. Think more of a series of firefights and clashes than one long brawl.