In a current German championship, a whopping 11 players currently have 5/6. The fact that 197 players are listed makes this a bit less remarkable, of course. Obviously, the probability p of such an event would be the most convincing record measure, but that's a tough one. I thus suggest the following approximation: Largest tie at the end of a 9 round swiss, measured by t=(tied people at the top)/(all people). (Or is there a statistical argument that I should apply some logarithms?) So we have about t~0.05 here, which should be easy to beat.
Anyway, what is the largest mass tie you recall? (Round robins would be also remarkable, but have the few players advantage in comparison here.)