2016-07-13 : If each Eldrazi that could block the equipped creature is tapped or is affected by a spell or ability that says it can’t block, then none of them block. If there’s a cost associated with having each of those creatures block, its controller isn’t forced to pay the cost, so they don’t have to block in that case either.

2016-07-13 : One creature equipped with multiple Slayer’s Cleavers only requires a single Eldrazi to block it.

2016-07-13 : Only one Eldrazi is required to block the equipped creature. Other Eldrazi and non-Eldrazi creatures may also block that creature, and are free to block other creatures or not block at all.

2016-07-13 : The defending player, not the attacking player, chooses which Eldrazi blocks the equipped creature.