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.

The defending player, not the attacking player, chooses which Eldrazi blocks the equipped creature.

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.

One creature equipped with multiple Slayer’s Cleavers only requires a single Eldrazi to block it.