My site with 10k uniques per day has 1,000,000+ pages, with non-unique content but it still does well. So you can imagine how well one with unique content and many more pages would do. Quora has 5 million pages indexed, Stack Overflow has 7 million indexed, Yahoo Answers – 37 million.
My site makes about $20 a day, not a lot, just $2 CPM since it has adult and duplicate content, so doesn’t qualify for Adsense. The questions are pretty obscure too and hard to monetize, so I’ve stuck with CPM networks.
So if you want to take this idea for yourself, I’d suggest focusing on unique, user-generated PG-14 content so you can monetize it properly. Then you’ll just need high question and answer volume and some backlinks to help you with ranking. If you don’t have many backlinks, you’ll likely only rank for obscure questions, which are harder to monetize.
Questions can be auto or bulk generated, but how do you get answers for them all? How can you incentive people to answer the questions? In the case of my site, the $7000 a year revenue won’t go far towards paying people to answer 1,000,000+ questions, so maybe that money should go towards marketing so that you can get lots of people coming, and then try to convince them to answer questions. Maybe offer prizes for random answers, which will be cheaper than paying for each answer.