Using a magical item to cast a spell

1) If the object uses your spellcasting modifier or your DC, it is simply a way for you to cast the spell without using a spell slot. This allows you to use any of your spellcasting abilities (metamagic, wild magic, spell sculpting etc.) In addition, it counts as casting a spell for the purposes of casting 2 spells.
Some Examples:

• Most staffs
• Necklace of prayer beads.

2) If the magical item specifies a DC or + to hit, then you cannot use any special spellcasting abilities and it is simply using an item action.
Therefore it does not count as "casting a spell" for the purposes of casting a 2nd spell. (Note: This is a home rule and subject to change.)
Some examples:

• All scrolls have a specific DC /+ to hit
• Most wands and magical objects fall into this category
• Necklace of Fireballs

Regardless of any spell cast or how it is cast, if concentration is required, you can only concentrate on 1 spell at a time. See Concentration spells