I realized recently that I know next to nothing about how many people were ACTUALLY doing the things often portrayed in pop culture about the era. I know much of the hippie revolution was centered in California and the West Coast, but not much beyond that.
One possible method is to look at surveys and see what people are saying. It's likely to be an underestimate, as people don't always want to admit what they're doing, but can show a trend over time.
You can see here it goes from 4% in 1969 to 24% by 10 years later.