NPC Knight ability “parry” – how often can it be used?

In the Monster Manual the Knight has a reaction ability called Parry:

Parry. The knight adds 2 to its AC against one melee attack that would hit it. To do so, the knight must see the attacker and be wielding a melee weapon.

I’m not clear on how often this can be used – it says “against one melee attack that would hit it” is this once per turn or just against any melee attacks?