"If you fight multiple opponents, this gets more complicated. With parry, the base chance is divided by the number of active melee opponents against you (if you had a 60% chance to parry, and are fighting 3 opponents, your chance is now 20% per attack)." http://talsyra.tripod.com/daocmechanics/weaponskill_mechanics.html
"Your chance to parry is affected by the number of attackers, the size of the weapon you're using, and your spec in parry.
Parry % = (5% + 0.5% * Parry) / # of Attackers
Parry: (((Dex*2)-100)/40)+(Parry/2)+(Mastery of P*3)+5
So, if you have parry of 20 you will have a chance of parrying 15% if there is one attacker. If you have parry of 20 you will have a chance of parrying 7.5%, if there are two attackers."
http://gaheris.net/phpBB3/viewtopic.php?f=37&t=730
"Your chance to parry is affected by the number of attackers, the size of the weapon you're using, and your spec in parry.
Parry % = (5% + 0.5% * Parry) / # of Attackers
Parry: (((Dex*2)-100)/40)+(Parry/2)+(Mastery of P*3)+5
So, if you have parry of 20 you will have a chance of parrying 15% if there is one attacker. If you have parry of 20 you will have a chance of parrying 7.5%, if there are two attackers."
http://gaheris.net/phpBB3/viewtopic.php?f=37&t=730