If you control more than one Craven Hulk, they can all block together, even if no other creatures block.

If an effect says that Craven Hulk blocks “if able,” and you control at least one other creature that is able to block, you must block with Craven Hulk and at least one other creature.

Once Craven Hulk has been declared as a blocking creature, it will remain in combat even if all other blockers leave the battlefield or are otherwise removed from combat.

Although Craven Hulk can’t be the only creature you declare as a blocking creature, it can be the only creature that blocks any given attacking creature. Other blocking creatures don’t have to block the same creature Craven Hulk does.