How many wagons do you need for arming 500 irregulars?

I am just DMing "Shadowdale – The Scouring of the Land" and would like to incorporate a caravan delivering arms to the dalesfolk. I figured 500 suits of leather armor, 500 large wooden shields, 500 shortspears and 2000 javelins would do. This amounts to roughly 20,000 lb. of weight. A heavy horse can drag 3,000 lb. Under favorable conditions (say: wheels) this is doubled (PHB p. 162), so 1 heavy horse drags 6,000 lb. So, if you reckon 2 horses in front of a wagon you would need only 2 wagons to transport arms for 500 light troops. That does not seem right. Is there a limit to the carrying capacity of a wagon?