You can roll a replenish modifier on magic / rare ones and as for Unique ones Titan’s Revenge also has that mod. They will slowly gain additional charges overtime.
Repair when you need to it also replenishes the quantity unless its ethereal.
For the unique one no it doesn’t just disappear when it hits 0 charges I don’t know for certain on the magic / rare ones that may just be something for white ones.
Also turn on your show ammo count in settings.