user11144: 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.)

user15223: There are a couple of records of this sort in the relevant Wikipedia page. To quote:

Thirteen players tied for first with 5–1 scores at the National Open held on March 17–19, 2000 in Las Vegas: grandmasters Jaan Ehlvest, Alexander Goldin, Alexander Baburin, Pavel Blatny, Eduard Gufeld, Yuri Shulman, Alex Yermolinsky, Gregory Kaidanov, Dmitry Gurevich, Alexander Stripunsky, and Gregory Serper, and International Masters Rade Milovanovic and Levon Altounian.[93]

There's also a double round robin record:

At the Linares 2001 tournament, five of the six players (83.3%) finished with a minus score. Garry Kasparov won with 7½/10, while Judit Polgár, Alexander Grischuk, Peter Leko, Alexei Shirov, and Anatoly Karpov tied for second to sixth places, each with 4½/10.


11144: <p>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.</p>
<p>Anyway, what is the largest mass tie you recall? (Round robins would be also remarkable, but have the few players advantage in comparison here.)</p>
 15223: <p>There are a couple of records of this sort in the <a href="https://en.wikipedia.org/wiki/List_of_world_records_in_chess" rel="nofollow noreferrer">relevant Wikipedia page</a>. To quote:</p>
<blockquote>
<p>Thirteen players tied for first with 5–1 scores at the National Open held on March 17–19, 2000 in Las Vegas: grandmasters Jaan Ehlvest, Alexander Goldin, Alexander Baburin, Pavel Blatny, Eduard Gufeld, Yuri Shulman, Alex Yermolinsky, Gregory Kaidanov, Dmitry Gurevich, Alexander Stripunsky, and Gregory Serper, and International Masters Rade Milovanovic and Levon Altounian.[93]</p>
</blockquote>
<p>There's also a double round robin record:</p>
<blockquote>
<p>At the Linares 2001 tournament, five of the six players (83.3%) finished with a minus score. Garry Kasparov won with 7½/10, while Judit Polgár, Alexander Grischuk, Peter Leko, Alexei Shirov, and Anatoly Karpov tied for second to sixth places, each with 4½/10.</p>
</blockquote>
