[RPG] cap to the number of charges a wand or staff can hold

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

The Detect Magic spell states:

If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).

Spellcraft states:

Identify the properties of a magic item using detect magic: DC 15 + Item's CL

So what, you might ask, is a "property?" Well, they don't define in a legalese way exactly what is included in a magic item's properties, except to note that it definitely gets you command words ("The spells detect magic, identify, and analyze dweomer all reveal command words if the properties of the item are successfully identified..."). One simply has to assume from general English definition and logic that it gives you anything beyond that, including what the item even does.

I think it's best to interpret "properties" as "All of what it does, including charges, command words, and whatnot. Its full rules stat block." (Excepting, of course, other defined exceptions like artifacts and spells on a scroll.) Analyze dweomer specifically says it gets charges, but relying on a 6th level spell to get the charge level of a plain old wand is pretty lame IMO.

In earlier editions of D&D I was fine with not telling people charges and letting them find out when they ran out - it added a nice randomization factor - but in Pathfinder where the Christmas tree syndrome tends to dictate that it's players' God Given Right to liquidate all treasure for a union-decreed cost to buy other gear, not knowing charges and thus value would be an impediment.