With the living population vastly reduced, there is likely plenty of ammunition for the remaining survivors. Ammunition is stored in places Rick's group is likely to loot anyway, making it easy to replenish as they go along: there is no need to stockpile thousands of rounds of various calibers. Since they kill the majority of the walkers using bladed and other melee weapons, they do not have a need for stockpiling (and carrying) tons of ammunition.