After decades of adhering to a tried and true formula, the Pokémon franchise has taken its first major departure in a mainline game. Pokémon Legends: Arceus makes a ton of major changes to how you will approach just about everything in this brand new adventure set in the far past of the Pokémon world. From exploration and combat to story and progression, there's a lot to come to grips with when starting out. But not everything old has been left behind. Items have been useful tools, or sometimes essential ones, since the original games. Many familiar ones are back again, but your new satchel doesn't work like the bags of previous games.
For the first time in the series, Pokémon Legends: Arceus limits how many items your character can hold at any one time. In past games there was nothing to stop you from loading up on 99 of most items. This could trivialize some battles, which this new limit on inventory space looks to resolve. Still, no one likes having to leave good items behind because they don't have the space, or run out at a key moment when just one more potion could make all the difference. Thankfully, there is a way to expand your satchel space in Pokémon Legends: Arceus, but it won't be cheap.
Further reading
How to get more inventory space in Pokémon Legends: Arceus
Before you have the option to expand your inventory slots, you first have to play far enough into the main campaign to unlock the Galaxy HQ Lobby area. This will happen within the first few hours, when you get the mission called The Frenzy of the Lord of the Woods, so just play as normal until it becomes available.
After you are automatically brought through the Galaxy HQ, an NPC named Bagin will interrupt you before you're able to leave. He will offer to give you more inventory space by teaching you how to more efficiently pack your bag. He will teach you this technique for a fee of 100 Pokédollars the first time, but each additional slot will increase in price each time you speak to him. At first the prices only go up by 100, but eventually increase to 500, 1,000, and eventual tens and hundreds of thousands.
You can upgrade your satchel a total of 37 times, with the final upgrade costing you you a full 1 million Pokédollars, so don't expect to max out your satchel right away. Bagin will always be there to teach you his ways, so revisit him as you make progress on your journey and build up funds.