[kobeya]

Depends on what you mean by "Stonehenge." The iconic stones are a late neolithic construction, but the site itself is very old. The visible portion today is built on top of a much older ritual structure.

I don't think you'd need 10,000 people to construct a structure like Stonehenge, especially when you consider that these were built over vast intervals of time (hundreds of years per structure, thousands for the entire site).