Rule 802.2 · Attack Multiple Players Option
As the combat phase starts, the attacking player doesn’t choose an opponent to become the defending player. Instead, all the attacking player’s opponents are defending players during the combat phase.
Subrules
Any rule, object, or effect that refers to a “defending player” refers to one specific defending player, not to all of the defending players. If an ability of an attacking creature refers to a defending player, or a spell or ability refers to both an attacking creature and a defending player, then unless otherwise specified, the defending player it’s referring to is the player that creature is attacking, the controller of the planeswalker that creature is attacking, or the protector of the battle that player is attacking. If that creature is no longer attacking, the defending player it’s referring to is the player that creature was attacking before it was removed from combat, the controller of the planeswalker that creature was attacking before it was removed from combat, or the protector of the battle that player was attacking before it was removed from combat. If a spell or ability could apply to multiple attacking creatures, the appropriate defending player is individually determined for each of those attacking creatures. If there are multiple defending players that could be chosen, the controller of the spell or ability chooses one.
Example: Rob attacks Alex with Runeclaw Bear and attacks Carissa with a creature with mountainwalk. Whether the creature with mountainwalk can be blocked depends only on whether Carissa controls a Mountain.