There are many, many more than 2,000 good ideas per year. The problem is that navigating human organizations (societies, markets, etc.) is very difficult. A good idea doesn't turn into a good business without the right combination of talent, timing, location, funding, etc.
So the issue isn't that there aren't enough good ideas. The issue is that there may not be enough teams matched with the right circumstances to fill out a successful cohort.
But the business model of venture capital isn't to come even close to getting are return from 100% of investments, so the question of whether there are 2,000 good companies a year is totally irrelevant. All YC needs to generate a return is to have 1 or 2.
