Ammunition should stack to at least 400. Special ammo such as doomshots and miniature cannonballs could stack further due to the way they're acquired.
Right now, the best quiver has 18 slots, lower than many high level bags like the Argent Dawn supply bag, and hunters are forced to always have one due to the haste buff. The best ammo pouch has 16 slots.
In contrast with normal Vanilla, ranged hunters consume many more arrows per second due to having to weave Steady Shot and Arcane Shot between ranged autos.
Increasing the amount of ammo stack would simply mean having to restock less often while spending the same amount of resources to acquire them so the overall time/cost ratio would remain the same.
Finally, add jagged arrows and accurate slugs to the vendors in Alah'Thalas (the city lacks many other higher level materials from vendors like soothing spices).