When one of my articles made it to the #2 spot on Hacker News, I was watching my Google Analytics Real-Time stats pretty closely. It got as high as ~700 people on at one point.
(Here's the submission: https://news.ycombinator.com/item?id=5062761)
Sat at a pretty constant 250 online visitors for hours. Pretty sure the brief peak must have been close to 700 as well.
Fortunately Webpop is built to handle that kind of traffic, so we never had to worry about the site crumbling under the load...
Though, now I'm remembering that the post was also on /r/webdev and /r/web_design, so that skews things a bit.
Luckily, I had recently converted the site from WordPress to Jekyll and was hosting it on Github pages, so I didn't worry too much about the load.