If a creature has more than one party creature type, and there are multiple ways to count that creature that could result in a different number of creatures in your party, the highest such number is used. For example, if you control a Cleric and a Cleric Wizard, the number of creatures in your party is two. You can't choose to have it be just one by counting the Cleric Wizard first as a Cleric.

To determine "the number of creatures in your party," check whether you control a Cleric, whether you control a Rogue, whether you control a Warrior, and whether you control a Wizard. The number is the total number of those checks to which you answered yes. Each creature you control can be counted for only one of those checks.

The cost reduction applies only once to any spell you cast that's a Cleric, Rogue, Warrior, or Wizard. A spell with more than one of these subtypes will still have its cost reduced by just {1}.

Some cards refer to you having a "full party." This is true if the number of creatures in your party is four.

The cost reduction applies only to generic mana in the total cost of Cleric, Rogue, Warrior, and Wizard spells you cast.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of The Destined Warrior's fourth ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.