How do the Prosthetic Limb and Wand Sheath magic items interact with the artificer's Soul of Artifice capstone feature?

Question

A player playing an artificer thought of an interesting interaction between the Prosthetic Limb or Wand Sheath magic items and the Artificer's capstone feature Soul of Artifice.

The Prosthetic Limb (Eberron: Rising From the Last War, page 278) reads, in part:

If you have multiple prosthetic limbs, they count as a single magic item with regard to the number of magic items you can attune to.

The Wand Sheath (Eberron: Rising From the Last War, page 279) states something similar:

However, the wand sheath and the attached wand count as a single magic item with regard to the number of magic items you can attune to.

The Soul of Artifice feature (Eberron: Rising From the Last War, page 58) includes the following benefit:

• You gain a +1 bonus to all saving throws per magic item you are currently attuned to.

The Prosthetic Limb and Wand Sheath say that they simply count as a single magic item with regard to the number of magic items you are able to attune to. However, it does not outright say that it changes the number of items you are currently attuned to.

Therefore, if you attune to multiple Prosthetic Limbs or a Wand Sheath with an attached wand, you will have attuned to multiple magic items, but it only counts as one for how many you can attune to. With Soul of the Artifice, it is possible to get multiple bonuses to your saving throws, despite the fact that you are only technically attuned to a single item.

By RAW, does these actually interact in this way?

Answer 1

Prosthetic limbs no longer require attunement

The November 2020 errata for Eberron updates the item description:

This item’s attunement requirement has been removed, and both paragraphs of the item’s description have been replaced with the following text: “This item replaces a lost limb—a hand, an arm, a foot, a leg, or a similar body part. While the prosthetic is attached, it functions identically to the part it replaces. You can detach or reattach it as an action, and it can’t be removed against your will. It detaches if you die.

Without attunement, there is no longer any interaction with Soul of Artifice to add bonuses to your saving throws.

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Below is the original answer which I am going to keep due to the previous voting. But please see above for the current answer that includes the November 2020 Errata for Eberron.

Those magic items don't change the attunement values.

You've quoted all the relevant bits, but the key aspect here is also what you've said: that the 'grouped' attunements count as a single attunement, and it is the attuned magic items that count towards the capstone feature.

Prosthetic Limb

This counts as a magic item attunement. If you've got multiple prosthetic limbs, they still just count as a single attunement.

So no matter how many limbs you've got, you're just using up one attunement slot and generating a +1 bonus because the rules state they count as one.

Wand Sheath

The Sheath has similar wording as to the limbs, in that sheath and wand count as a single magical item for attunement, and therefore a single magic item with regard to the Artificer capstone.

But what about the fact that you've got multiple items? Shouldn't that increase the bonus?

Well, no, not really. I mean, the language is kind of loose, but the capstone refers to magic items attuned, not magic items total. And when each item like the prosthetic limb or wand sheath specifically states that they count as a single attunement, then it really is just that.

Answer 2

The text is clear.

Your attuned items are what gives you a bonus.

You have multiple attuned items that count towards your attuned item limit as if they were one. Your multiple items themselves are still attuned.

As written, its indeed a loophole. Up to you if you want to assume its against the intended spirit.