[RPG] When casting a spell using Twinned Spell does it use one or two spell slots

You can apply class abilities to spell casting from magical items

While casting from an item is not the same as a Casting a Spell action, it is still casting a spell. Unless a term is given a special definition in 5e game terms, you use the common definition for that term. You are casting the spell, the ability to do so comes from the item. For instance, the Staff of Fireballs:

.. you can use an action to ... cast one of the following Spells ...

Is no different, in this regard, to Wand of Fireballs from:

you can use an action to ... cast the Fireball spell ...

Jeremy Crawford has posted a number of answers we can point to in the time since this question appeared that verifies this really is the case. Each time he's been asked about a specific class ability the answer has been an affirmative. These were made when his tweets were official rulings, but since have been "downgraded" by the Sage Advice Compendium to his personal advice and perspective. While this is true, it still is clarification of the intent of the designers.

Metamagic

Can Metamagic be used on magic item casting a spell? Yes.

If a magic item's description says you cast a spell from it, you can use Metamagic on the spell. #DnD

and

Metamagic works w/ any spells that sorcerers cast. Wild Magic Surge can work w/ any sorcerer spell they cast. #DnD

Wild Magic

Can using a magic item cause a Wild Magic Surge? Yes

Yes.

Arcane Ward

Can casting from a wand/staff recharge Arcane Ward? Yes

Arcane Ward/Twinned Spell works when you cast a qualifying spell. It even works when an item says you cast one. #DnD

Two slots -- one for the stored spell, and one for the Glyph (which must be the same level slot (or higher) as the spell being stored).

The Spell Glyph option of the Glyph of Warding spell (PHB, p. 246-247) states:

You can store a prepared spell of 3rd level or lower in the glyph by casting it as part of creating the glyph. [...] The spell being stored has no immediate effect when cast in this way. When the glyph is triggered, the stored spell is cast. [...]

Both slots are expended when the Glyph with the stored spell is created, as that is when the spell caster is casting both spells.

There's nothing in the spell description that says it allows an extra spell to be cast and stored without using a spell slot. Using a spell slot is how casting and storing spells both normally work (per p. 201 of the PHB), so that's the general rule, and it holds unless a specific rule says otherwise for a particular application - which in this case it does not.