Some missions can have low ammunition as a descriptor. Also low ammunition is on the random complications table in the GM section.
I would assume they mean virtually unlimited normal ammo, but reloads are required. I would assume each weapon could be reloaded at least three (3) times.
Examples:
A) A bolter and nine (9) magazines would be standard (three are loaded at a time).
B) A bolt pistol with thee (3) magazines for all but an Assault Marine would have twelve (12) magazines, perhaps twenty-four (24) magazines. Remember bolt pistol clips hold half the bolts compared to a bolter clip.
C) A heavy bolter would have the backpack with 250 rounds and three (3) 60-round clips.