During the League of Legends Quarterfinals in Chicago on Friday, viewership of the SK Telecom vs RNG game peaked at 619,498 across all active channels on Twitch (if anyone has Youtube data, feel free to submit!).
Data was extracted from the Twitch API from TwitchApps which polls the Twitch Servers every 10 seconds and averages the result.
The second most popular match was the Albus NoX Luna vs H2K match which brought in a recorded average peak of 552,016 viewers followed by the Cloud 9 vs Samsung Galaxy game at 523,560 and Rox Tigers vs Edward Gaming at 501,065.
LCS Quarterfinal Viewership
Thurs: 523,560 Cloud 9 vs. SSG
Fri: 619,498 SK Telecom T1 vs. Royal Never Give Up
Sat: 501,065 Rox Tigers vs. Edward Gaming
Sun: 552,016 H2K vs. Albus NoX Luna
** These numbers include non-riot channels (necessary given the way the data is aggregated), but assume that a majority of league viewership during quarterfinal matches was devoted to the LCS main or secondary channels.
Due to TwitchApps polling methodology, the peaks may have been slightly higher, though if we factor in and subtract the included streams that were not focused on Worlds then the numbers reflected are likely closer to the real peak.