Data on how large a particular Magic the Gathering tournament was?
Further exploring the link you provided, it seems that, after selecting an individual event, you can see it's Coverage by days.

Among that information the number of assistants is reported. For example, on the United States National Championship 2018 event it reads on the first paragraph (emphasis mine):


  It was a weekend that comes only once a year – US Nationals, when players compete for the title of National Champion and a spot on the US World Magic Cup Team. On Saturday, 739 players arrived at the Greater Columbus Convention Center, ready and eager to play for their chance at the title of US National Champion.


The same seems to apply for any of the events they list, like this other, Sao Paulo's Celebration of the 25th anniversary:


  1,429 players gathered in São Paulo to celebrate Magic's 25th anniversary, making this the largest tournament South America had ever seen. Needless to say, they also came to play thousands upon thousands of matches of Modern!