1) YC's acceptance rate for applications has remained basically constant since 2015 (< 2%). This batch is not bigger because we accepted a higher percentage, but because we got way more applications. The primary reason we got more applications is that over 15K companies participated in our startupschool.org program and many of them applied to YC. Our general policy at YC is to grow the batch size proportional with the number of good applications we get, and this larger batch is just a natural consequence of that policy.
2) We have been wanting to move demo day to a new larger venue for a long time now. The reason is not that there are too many startups, but that there are too many investors who want to come. We have enough demand from investors to fill the previous demo day auditorium several times over, and this new space means we can allow many more investors to come, which is better for the companies.
3) As the batch at YC has grown, we've grown the number of partners proportionally and plan to continue to do that. The ratio of companies to partners is actually lower now than it was a few years ago, and we've increased the amount of time we spend per company.
4) I think the ultimate measure founders should care about - and certainly the one we do - is the success rate of YC companies on average. If we grew the YC batch and the success rate went down, that would be bad. So this is something we track extremely carefully to make sure that does not happen. And so far, it hasn't. Based on the last 1-2 years of companies, the success rate of YC companies is the highest it's ever been.
Also for general background, I'd really recommend reading a great short statement Paul Graham wrote about why we think growing YC is better for both us and the founders: https://www.ycombinator.com/atyc/#size