Deadly Alliance — ruling 1

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.

Deadly Alliance — ruling 2

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.

Deadly Alliance — ruling 3

An ability referring to the number of creatures in your party gets a number from zero to four. Such abilities never ask which creatures are in your party, and you never have to designate specific creatures as being in your party. You can't choose to exclude creatures from this count to lower the number.

Deadly Alliance — ruling 4

Several cards have a cost reduction based on the number of creatures in your party. To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

Deadly Alliance — ruling 5

If a spell has a cost reduction based on the number of creatures in your party, no player may attempt to change that number after you begin to cast the spell but before you pay the cost.

Deadly Alliance : règles, fonctionnement et interactions (MTG)

Deadly Alliance is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs less to cast for each creature in your party. (Your party consists of up to one each of Cleric, Rogue, Warrior, and Wizard.) Destroy target creature or planeswalker.

How it works : Deadly Alliance

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#2September 25, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#3September 25, 2020· Wizards of the Coast
An ability referring to the number of creatures in your party gets a number from zero to four. Such abilities never ask which creatures are in your party, and you never have to designate specific creatures as being in your party. You can't choose to exclude creatures from this count to lower the number.
No other card is named in this ruling.
#4September 25, 2020· Wizards of the Coast
Several cards have a cost reduction based on the number of creatures in your party. To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
No other card is named in this ruling.
#5September 25, 2020· Wizards of the Coast
If a spell has a cost reduction based on the number of creatures in your party, no player may attempt to change that number after you begin to cast the spell but before you pay the cost.
No other card is named in this ruling.