I have 14 items in my L3 magic shulker, but I know for a fact that all 14 are not functioning, despite the wiki stating the first 16 slots should work: the B3 Magic Spruce Item is for sure not working when placed in the 15th slot (where I had it originally), worked when placed in slot 12. and didn't work when placed in slot 13. After some moving items around, it seems the L3 shulker only works on the first 12 slots and not 16, so either the skulker isn't working correctly, or the wiki is incorrect Other magic items in the L3 shulker don't work past slot 12, so it can be tested/confirmed with any item. I just used my spruce.
According to the two relevant wiki pages, the magic shulker's stages (from Magic I-III and Boost I-III) are 4 - 8 - 16 and 16 - 20 - 24. I suspect this is a typo on the magic wiki page and that it is intended to be 12 for Magic III.
The code states: Code: int maxItems; if (magicLevel == 1) { maxItems = 4; } else if (magicLevel == 2) { maxItems = 8; } else if (magicLevel == 3) { switch (boostLevel) { case 1 -> maxItems = 16; case 2 -> maxItems = 20; case 3 -> maxItems = 24; default -> maxItems = 12; } Meaning 4 8 12 for Magics. Meaning 16 20 24 for Boosts. Potential typo by @ClarinetPhoenix on the wiki.