There are no consumable items in XCOM:Chimera Squad (or if there are, I did not encounter any). Items and abilities can be limited to:
- Once per x turns (cooldown)
- Once per encounter
- Once per mission
Any equippable items will be available again after the mission is over. So there is no reason to not make use of them, especially during the last encounter of a mission. And there is no reason to buy more than one of each either, unless you intend to bring more than one of them on the same mission (usually you don't, because there are just so many different items available)
Grenades are usually once-per-mission, but the agent Claymore can learn the optional ability "Improvised Explosives" on his 2nd promotion which upgrades grenades in his inventory to once-per-encounter.
Breach-items like breaching charges and key cards are all once-per-mission. But it is pretty rare to encounter the same kind of item-accessible breach point more than once in a single mission. Further, you often don't need to bring a key card, because those agents who are accompanied by a Gremlin drone (Terminal and Patchwork) do not need a key-card to access keycard-accessible breach points. So you usually should carry one breaching charge at most and use the breach item slots of all other agents for items which provide a bonus during the breach.
Special ammunitions are not limited at all. They apply to all shots fired by that agent while equipped.