Hahah, I always get a good giggle when I hear this.
Let's go with data from
http://cdn.themis-media.com/media/gl...117/117205.png
With 1 000 000 000 hours played per month it comes out to around 1 700 000 000 games per month if the average game lasts 35 minutes. That's about 55 mln games per day. Let's assume 0.1% of these contain a troll and somehow riot employees mystically know which ones are those, that leads to 5500 games per day to study. At the average 35min used earlier that's 3200 hours of games to watch per day. That sounds reasonable, eh? I mean, only a bit over 400 employees dedicated to watching those games throughout entire 8-hour shift would be enough!