What happens when a card says you need two creatures to defend an attack?

Question

If a player attacks with Pyreheart Wolf which states

each creature you control can't be blocked this turn except by two or more creatures

how is damage calculated, or what happens more generally?

Answer 1

The answer to your question as written is that Pyreheart Wolf itself makes no difference to how damage is calculated.

Specifically, Pyreheart Wolf creates a restriction in terms of Comprehensive Rules (CR) 702.111b and 509.1b that the attacking creatures cannot be blocked except by two or more creatures:

702.111b A creature with menace can’t be blocked except by two or more creatures.

509.1b The defending player checks each creature they control to see whether it’s affected by any restrictions (effects that say a creature can’t block, or that it can’t block unless some condition is met). If any restrictions are being disobeyed, the declaration of blockers is illegal.

Once a legal declaration of blockers has been made, the game then proceeds as it always would through the players announcing damage assignment orders (CRs 509.2 and 509.3) and then to the combat damage step (CR 510). There is no difference in how damage is calculated because Pyreheart Wolf created a restriction; exactly the same process is followed as it would be if the defending player chose to block an attacking creature with two blocking creatures when no restrictions were in effect.

(Aside: why am I referring to CR 702.111 "Menace" when Menace is not in the text printed on your card? The answer to this is because Pyreheart Wolf has been updated since it was printed, and Magic: the Gathering always uses the current "Oracle text" for a card as can be found at Gatherer; as you can see there, Pyreheart Wolf's text is now "Whenever Pyreheart Wolf attacks, creatures you control gain menace until end of turn.")

Answer 2

When more than one creature blocks an attacker, damage works like this:

1. If any of the creatures have first strike, that damage is dealt first. Any other effects happen (e.g. lifelink, deathtouch). If any creatures die at this point, then they go to the graveyard and do not participate in the combat anymore.
2. All surviving creatures then deal damage during the normal combat phase. All defending creatures do their damage to the attacker, while the attacker splits its damage against the defenders.

Example:

Suppose I attack with a 4/5 and you block with a 3/3 and 4/4. My 4/5 has first strike, while neither of your creatures do. The rules say that I need to "order" your blockers - that is, choose whether to do damage to the 3/3 first, or to the 4/4.

In the first scenario, I do 3-4 damage to the 3/3 and 0-1 damage to the 4/4 (most of the time there is functionally no difference, but if it matters, I get to choose). The 3/3 dies, the 4/4 stays alive. During the normal combat phase your 4/4 does 4 damage to my 4/5, which isn't enough to kill it (barring special effects like deathtouch). Combat ends with both of us still having a creature.

In the second scenario, I must do 4 damage to the 4/4 and 0 damage to the 3/3. This is because I must always kill the first blocker before doing damage to the next blocker. The 4/4 dies, the 3/3 stays alive, and you do 3 damage to my 4/5.

Note I (the attacker) am the one that gets to choose between the two scenarios. You only get to choose to block with 2 creatures.