There are some pole like things with a screen on top which are scattered around the map. They are called Depots.
Use box [ ] button when near them to open their menu.
Select "Ammo Refill" from the available options.
It requires 100 CP per refill.
CP is earned during match.
Do this for 10 times total. ( Not necessary to do in a single match )
Trophy unlocked for me only after completing the match.