I would say it's closer to 10-15 spaces, than 15-20.
If we add up the total amount of different items you can have in your inventory, in order to make progress, we have:
Bone Dust, Undead Essennce, Undead Energy, Spirit Orbs, Loyal Spirit Orbs, Bright Auras, Brilliant Auras, Blinding Auras, Blinding Light Fragments. So 9 items needed to turn in quests and merge stuff.
6 different weapons, each one representing a different metal, so I won't count metals as well, it would be pointless.
Basic, Advanced and Ultimate Weapon Kit.
All of this adds up to 18, plus 2-3 more for picking up optional items to stop them from dropping, the ones that comes to mind are the Treasure Hints from Undead Pirates, Dage's Favor from Battleunderb and Treasure chests. You can cut down to only 3 weapons (and only 1 when you don't need the others at the same time), cut all weapon Kits, because you can merge them while you have low inventory usage, and you will never be farming Blinding Fragments and Bone Dust + Essences at the same time. Already here, we have cut it down to just about 10 spaces (assuming you only farm with 1 Blinding Weapon in the inventory) and being able to comfortably farm everything at once.
In theory, you can make do with 7 spaces, which allows you to merge and turn in every different quest, but it would mean you have to go to the bank a lot more than lets say 10-15 spaces.
With all that being said, you will "only" need 10-15 (15 meaning you have a lot to spare) spaces to farm comfortably, which can be less than 10, if managed correctly. It's actually fairly generous in comparison to most higher tier Nulgath items.
As Icetex has already mentioned, the easiest way to start off is to open the "challenge shop" in Doomwood by killing 100 Undeads on that 1 specific screen of Doomwood. Once you have done that and gotten past the initial 2 quests, then the main grinds starts, which consists of farming Bone Dust and Essences in /Battleunder, or the "Soul Searching" quest from Yara in /Battleunderc until you have what amounts to 105 Loyal Spirit Orbs (or 10.5k Spirit Orbs). Then you can progress to the next step of making your first "Metal of Destiny" which is needed to merge the different weapons that will greatly increase your farming speed for Orbs and Auras.
< Message edited by Metakirby -- 3/16/2018 10:46:09 >