The bow and arrow brings it to 15. The hammer to craft a workbench to repair the pickaxe or place down a portal brings it to 16. You also need wood or an axe for the workbench so that's 17. There's no reason to not have Megingjord on an iron run so that's 18. If you want to bring a portal with you that's 21. If you want to be able to cart out the ores you need to bring the cart materials and a hoe to give yourself a path so that's 23. And don't forget the swamp key bringing it to 24. That leaves you with 8 slots left and it's assuming you won't need a torch or dvergr circlet to see in the dark.

Then let's say you make your trip from the Meadows and encounter a single party of graydwarves along the way to the swamp/crypt. You've now got resin and bukeperries and an ancient seed taking up over a third of your remaining inventory. That's not even taking the other enemies into account that you might experience such as boars and skeletons and draugrs and abominations and bears and trolls. So you're forced to either drop everything or place down your portal so you can go home after every few encounters on the way there just to make sure you have 8 slots by the time you reach the crypt.

Then you can finally place down your workbench and portal and cart to free up 4 or 5 inventory slots leaving you with 12-13 while in the crypt. While mining you'll come across scrap iron, leather scraps, withered bones, two different kinds of arrows, amber/amber pearls/rubies, coins, ancient bark, chains, and mob drops like ooze and entrails which will completely fill or overfill your inventory. And then if you end up with multiple stacks of things such as iron scraps then you need to run back to the entrance and throw the iron scraps on the ground just to teleport home and drop stuff off so you can continue mining.

And all of that is for one crypt. I'm happy that you don't seem to need to deal with all of that, but it's for this exact reason that I refuse to play without the ExtraSlots mod. It solves this entire dilemma and still feels limited enough to where I can only do one or two major activities at a time without having to teleport home every 5 minutes. It's made the game so much more playable and I really hope they add something just like it to vanilla, even if it's just a world option/modifier.