So to complete the thought, the Bay Area has about 5x the average density of programmers.
Silicon Valley as a measure - 1% of the country's population but 10% of its programmers. So, 10x the national average of programmers.
Here's a relevant page by the Bureau of Labor Statistics. If you scroll down a bit past the state-level charts, they start showing metropolitan areas. "San Jose-Sunnyvale-Santa Clara" has a fairly good compromise between volume and density, but it's not at the top of either chart.