The term "defending player" in the myriad rules (or any other ability of an attacking creature) refers to the player the creature with myriad was attacking at the time it became an attacking creature this combat, the controller of the planeswalker the creature was attacking at the time it became an attacking creature this combat, or the protector of the battle this creature was attacking at the time it became an attacking creature this combat.

If the defending player is your only opponent, no tokens are put onto the battlefield.

You choose whether each token is attacking the player or a planeswalker they control as the token is created.

Although the tokens enter the battlefield attacking, they were never declared as attackers. Abilities that trigger whenever a creature attacks won't trigger, including the myriad ability of the tokens. If there are any costs to have a creature attack, those costs won't apply to the tokens.

The token creatures all enter the battlefield at the same time.

Each token copies exactly what was printed on the original creature and nothing else. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the copied creature will also work.

If myriad creates more than one token for any given player (due to an effect such as the one Doubling Season creates), you may choose separately for each token whether it's attacking the player or a planeswalker they control.

The ability that defines Polygoyf's power and toughness works in all zones, not just the battlefield.

The ability that defines Polygoyf's power and toughness counts card types, not cards. If the only card in all graveyards is a single artifact creature card, Polygoyf will be a 2/3. If the cards in all graveyards are ten artifact cards and ten creature cards, Polygoyf will still be a 2/3.

Card types that can appear on cards in a graveyard are artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Lhurgoyf, Forest, and Siege are subtypes, not card types.