My guess would be a few thousand in the Bay Area. Crunchbase could probably give you a better number by doing a bit of data mining on their database to find former Sun, Yahoo, Amazon, Facebook, Tesla, Apple, and Google employees who have gone on to create startups and exited those startups. Those companies alone have minted > 10,000 multi-millionaires who were simply employees at the company either prior to an IPO or prior to a large run up in value (like Apple).

Of people I know who have stepped off at stage 2, they pocket enough wealth that they no longer have to work to maintain a moderate lifestyle, and then move to a place that interests them and work on other pursuits.