Can anyone explain how does YC's business model works? They seem to keep adding several more partners often. I would guess the current toll should be more than 50. Do these partners also invest their own money in YC or no?
As far as I know, the partners help choose the startups to fund and advise them. I don't think they invest their own money (except maybe after Demo Day[2]), but choose how to invest YC's money at the start of each batch.
[1] http://www.ycombinator.com/partners/
[2] http://blog.ycombinator.com/new-yc-partner-investment-policy