Not many, and they would mostly be acqui-hires or early hires. Google strongly prefers candidates from a list of preferred universities. If you didn't graduate from one, you'd need to be coming from another employer in order to get hired there (and even then, my understanding is that those with degrees from those universities have a better chance of being hired, all other things being equal).

They also hire (or at least recruit) based on Open Source work (I've gotten recruitment letters from them based on a book I wrote and some Open Source software I've been involved in), but I doubt that I would have been hired just on the strength of that. I've never taken them up on the offer of an interview, as I liked working for myself too much, but I suspect I would not measure up during the process, given my self-taught background and unimpressive education history. I know several folks who have been hired by Google, including folks who got the recruitment letter based on Open Source software work, and they tend to also have the strong college background.

In short, I believe that for those folks who end up at Google, they very likely got there by way of a good university. So, the upstream needs to be fixed for the diversity at Google to be fixed.