Your character's layout is usually a lot more restricted as compared to that of a storefront. However, the character's layout needs to follow the same design pattern you are using in your game.
These systems usually fall into a couple of patterns.
Players are limited to a certain number of each item. The number can vary based on the item (for instance, you can have only one weapon) or its effect on the player's load. As a rule of thumb, 99 should be the maximum number. However, it's up to how your game will use the item to denote its maximum number.
In the Final Fantasy series of games, Rule of '99 was used throughout its inventory system, allowing the player to carry no more than 99 potions at a time or anything else for that matter, as shown in the following screenshot:
