Ryuutama tracks inventory with a combination of slots and containers, and it can sometimes be tricky to figure out exactly how much you can carry. Here’s a sheet to help you visualize it.
The left column counts your actual carrying capacity. Use the right columns to show the space you get from containers. e.g. a Large Backpack takes up 5 carrying capacity, but gives you 10 slots, so block out 5 rows and extend them two columns wide to get a total of 10 slots.