Wellington’s army had 156 guns at Waterloo.
Those guns formed the artillery arm of the Anglo-Allied army commanded by the Duke of Wellington. The army included British, Dutch-Belgian, German, Nassau, and Brunswick contingents, so its artillery was multinational as well. Guns were positioned to support the defensive line, cover approaches to fortified farms, and fire on French infantry and cavalry.
Artillery was especially important during the French cavalry attacks. Wellington ordered many gun crews to take shelter inside the infantry squares as cavalry approached, then return to their pieces when the horsemen withdrew. This allowed the batteries to resume fire between attacks. Captain Mercer’s battery famously continued engaging the cavalry from close range, although his account and Wellington’s later comments differ over the conduct of some artillerymen.
The 156-gun figure refers to Wellington’s army, not the combined Allied and Prussian forces that eventually fought at Waterloo. Napoleon’s main army had its own larger artillery establishment, while Blücher’s Prussians arrived with additional guns during the battle.