Mike,
From Braddock's March, describing the flying column (page 180):
"St. Clair was to have his standard armament of two 6-pounders, plus three wagons of clearing tools. The main party of the detachment, under Braddock, would carry four howtizers with fifty rounds each, the four 12-pounders with eighty rounds each, three cohorn mortars, and thirteen wagons, one of which bore presents for the Indian and the others ammunition. The column's packhorses would carry thirty-five days' food and provisions. Braddock increased the horsepower of the wagons and gun carriages to elevate speed. He now assigned six, rather than four horses to each wagon. And he increased the complements of horses for the cannon to seven for the 12-pounders and nine for the howitzers. An extra one hundred replacement horses joined the four hundred specially chosen for the column."
Dunbar's supply column would have had additional horses beyond these 500.
I find it interesting that the 8-inch howitzers needed 9 horses while the 12-pounders only needed 7. Guessing the howitzers are suprisngly heavy.