Perhaps less than we think.  According to George Barna most don’t make the cut as the NEA defines an evangelical.

A relatively substantial number of
people label themselves as evangelicals. But new research by The Barna
Group found a much smaller number of people actually fit the criteria.

For two decades, The Barna Group has been measuring the characteristics
of evangelicals based on a 9-question set of criteria. In its latest
study, the research group names them "9-point evangelicals."

On a general note, 38 percent of the population accepts the label of
evangelical. When The Barna Group measured them using its nine
questions to categorize evangelicals, only eight percent of the adult
population fit the criteria.

Applied to the entire adult population, the difference is "staggering,"
the report stated. Self-proclaimed evangelicals would number 84 million
versus 18 million 9-point evangelicals.

                   

and

On another note, the study found that 27 percent of those who say they
are evangelicals are not born again, based upon their beliefs, The
Barna Group reported, and they are also less likely to read their Bible
and attend church during a typical week.

Very interesting research, huh?

Source: http://www.christianpost.com/article/20070117/25224_Study%3A_Most_%27Evangelicals%27_Do_Not_Meet_Criteria.htm