(I know it's better to provide references to web pages and not books, but I don't have a web page that recreates that book on hand. Suggestions welcome.)

As I understand it, it's an estimate, not an actual figure. In order to perform such a survey, we'd have to sample people, record their entire daily activity, and determine how many crimes were committed. Determining how many crimes were committed would require an army of lawyers and private investigators... and that's the real point. As a normal citizen without an army of lawyers, you should have no confidence whatsoever that you are not committing crimes. If measured by "what could be used to convict you if the government wanted you out of the way" I'd guess 3 per day is a grotesque underestimate, at least one order of magnitude and I wouldn't bet much against 2.