A fighter has 100 OB; he's attacked by 3 other fighters. He parries with 50. Does this parry apply to each of the three enemy attacks or just to one attack of one enemy fighter?
MarcR described the RAW, I'll describe an option I use.
Parry must be declared before init, but parry is applied to all frontal attacks. Parry becomes a pposition of defence. There are a few things to note if the option is used.
Shield can still be applied against one foe only.
After being attacked by anyone, player may not abort action and is dedicated to the assumed defensive position.
Attacks require minimum 80% activity.
I have used this option for many, many years with no balance issues. Being flanked or attacked from behind remains dangerous and PC's should mnv to avoid it. This option is more cinimatic than the RAW and allows for PC's to survive multiple foes/encounters UNLESS they get flanked, at which point they are gonna get hurt (just as it is normally). I adopted this rule because in my experience, if a fighter has room to mnv and can keep his foes in front of him, he can defend himself against more than one foe.
Another issue you will run into is mnv'ing to NOT be engaged. There is plenty of talk about disengaging from an enemy (and the rules are clear enough IMO), but mnv'ing to not be engaged while fighting others is another issue, one that becomes very important when foes attempt to overwhelm a PC's defenses by bum rushing him with four or more (I assume the front of a PC consist of the front three facings on a hex grid and sheild DB can be gained against any foe making a frontal attack). To avoid such attacks I demand 10% activity and then roll a mnv, difficulty based entirely on GM fiat. % result is chance foe is prevented from engaging. Note that PC requires adaquate room to move to avoid attackers. Any PC without sufficient room should be smart enough to surrender, likewise if he is facing guards with bow support.
One cool thing that occurs from time to time using these rules to avoid being engaged is the "mad dash across a room to grab something and flee." For each foe that is to be avoided, 10% activity is required, which of course lowers distance PC can travel, and each for requires a mnv. I typically limit the pace of such a mnv to a sprint (this is a house rule of mine: pace above sprint requires to much concentration and effort to allow most mnvs outside of simply running or charging...if players don't buy this arguement, start using ext points and they will quickly see the error of their ways).