With the exception of Eggs, all items you’ve collected from leveling up and gathered from PokéStops will appear in your Bag. Access your Bag 
from the Map View by tapping the Main Menu 
button. By default, your Bag can hold an inventory of 350 items. You will not be able to gather new items from PokéStops when your Bag is over the limit. You can free up your inventory by using or discarding items. Purchasing an Item Bag from the Shop increases your inventory capacity. 
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Pokémon GO Game GUIDE