Cold Iron: When you score a critical hit against a target benefiting from spells that grant temporary hit points or bonuses to AC, ability scores, attack rolls, damage rolls, or saving throws, you must immediately attempt a special dispel check (1d20 + your base attack bonus) as a targeted dispel as per the dispel magic spell, but targeting only those spells. Once a creature has been the target of this effect, you can't affect it in this way again for 24 hours.
Does this try and dispel every defensive spell buff on the target or just a single buff? I assume just one.
"As a targeted dispel". To know what this means, you need to reference the spell.
Targeted Dispel: One object, creature, or spell is the target of the dispel magic spell. You make one dispel check (1d20 + your caster level) and compare that to the spell with highest caster level (DC = 11 + the spell's caster level). If successful, that spell ends. If not, compare the same result to the spell with the next highest caster level. Repeat this process until you have dispelled one spell affecting the target, or you have failed to dispel every spell.
Just one buff is devoured, the one with the highest caster level the dispel check allows you to beat. In theory, if there are ties (such as if the caster cast his own buffs), the GM chooses which one is dispelled randomly.
Note, also, that if you are aware of a specific buff on the target, you can attempt to dispel that buff and only that buff.