"each time the bearer fights, it can make 1 additional attack with each dread klaw it is equipped with."
To answer for those who might still have the same question, additional Dred Klaws grant additional attacks.
Note how the rule does not say the likes of "each time a model attacks using this weapon." This means that you are essentially paying for +1 A per dread klaw.
Deff dread has base 2 attacks, which goes up to 4 attacks during fight phase thanks to the two dread klaws it's equipped with. Any additional dread klaws beyond the initial two grants you 1 additional attacks in the fight phase.
If you have 3 dread klaws, you have 5 attacks. If you have 4 dread klaws, you have 6 attacks.