If multiple creatures block Anzrag, the Quake-Mole, its first ability still triggers only once.
Anzrag, the Quake-Mole
If you activate Anzrag, the Quake-Mole's last ability, but each creature the defending player controls can't block for any reason, Anzrag isn't blocked. If there's a cost associated with blocking Anzrag, the defending player isn't forced to pay that cost, so it doesn't have to be blocked in that case either.
If you activate Anzrag, the Quake-Mole's last ability, only one creature is required to block it each combat this turn. Other creatures may also block it and are free to block other creatures or not block at all.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Helge C. Balzer
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