For future reference, this kind of question is an excellent candidate for the stickied Short Answers to Simple Questions threads.

The amount carried by individual soldiers varied, but it was usually between thirty and sixty rounds, often split between cartridge boxes, knapsacks, and pockets when necessary. Standard cartridge boxes of the time could hold 35 charges. During the Jena campaign, soldiers carried 15 rounds in their packs and 35 rounds in their cartridge boxes, for 50 total. Davout's 1st Corps during the invasion of Russia split their sixty rounds between a cartridge box on the hip and spare rounds in the knapsack.

In sustained combat, this would be enough for about one hour of continuous fire. Considering some battles lasted three or even four days, this might not sound like much, but it's important to remember that during the Napoleonic Wars, the emphasis was increasingly on keeping troops out of action until the last possible moment. As such, only a small portion of the infantry would be taking part in the general firefight at any one time.

However, depletion of ammunition was a serious issue for units in the firing line. Clausewitz notes with some frequency the importance of soldiers having fresh ammunition; after a successful engagement, the victorious troops are often vulnerable to counterattacks, having shot off much of their ammunition. Part of preparing these units to resume combat was distributing ammunition. For this purpose, divisions normally had a few caissons full of musket cartridges to distribute to front line units and top them off. A division of 8000 might have as many as 160,000 spare rounds in the caissons, or an average of twenty per soldier. As a result, it was possible for soldiers heavily engaged to expend as many as one hundred rounds per man.