Conveyance of Small Arm Ammunition.

5. The usual mode of conveying small arm ammunition in the British service has hitherto been the musquet ball cart, holding 12,000 rounds, and drawn by two horses. During the Waterloo campaign, however, only 10,000 rounds were carried in the cart, as that quantity was deemed a sufficient load; but this, in common slow movements, even was found too much for a pair of horses: far less could they be expected, therefore, to move at an accelerated rate, when such was necessary.

In the Peninsula, where it was an object to take forward as great a quantity of ammunition as possible, the cart carried the 12,000 rounds; but to insure its getting on, there was a necessity for its being drawn by four horses, and they would have been required in France, on account of the deepness of the cross roads, had the cart been loaded to its full extent.

Minutes of Proceedings of the Royal Artillery Institution, vol. 1, p. 223.
https://books.google.com/books?id=5_hBAAAAYAAJ&pg=PA223

For the French, normally 16,000 rounds per caisson (more precisely, 16,335 for 8-pounder and 12-pounder caissons, 13,935 for 4-pounder).