If multiple creatures block Anzrag, the Quake-Mole, its first ability still triggers only once.

Anzrag, the Quake-Mole's first ability may trigger multiple times in the same turn. Each time it resolves, you create an additional combat phase.

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.

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.