To change the max amount of ammo a magazine can store go to the ItemList and select the magazine item row. Then in the Container Details you will find the Item Count Limit. Here is where you can set the max amount of ammo a magazine can store.
You can also change/set the Restriction Items this allows you to set which ammo items can be loaded into the magazine. You can set as many different items here as you like. Just click the little + button to add more or the trash can button to remove one. For this to work correctly make sure that Container Restriction Type is set to Allow Items.
You can find out more about container items in the video (here)
To change the amount of ammo that can be chambered into a weapon you will need to go to the ItemList and select the weapon item row, then in the Container Details you can find the Item Count Limit and change that value. To change the allowed ammo types you can add or remove items to the Restriction Items like explained above.