Tap on your Backpack, the centre icon on the Map, and then Inventory.

The Inventory is where you can see your:
  • Grenades
  • First Aid Kits
  • Flares
  • Materials to build Safe Houses
  • Upgrade Tokens (to unlock weapon upgrades)
  • Training Tokens (to unlock new Companion abilities)
Each item has a maximum amount, and upgrading specific Perks can increase that limit.