I've been crunching through the SBA's annual Small Business Economy report. They have an interesting table on self employment showing that about 10% of Americans are self employed (table 1.3 on page 20).
Going to the more detailed data on Table A.13 on page 318, we find that total number of self employed is 15,739,000 and that the number of self employed grew by 13% between 1995 and 2005.
Other interesting data is that hispanic self employment is up almost 100% during that period and self employment among those 55-64 increased roughly 47%. Both populations grew during the period,but the numbers are still quite impressive.
The SBA also has a working paper on this topic that reports that individuals with more schooling are more likely to choose self employment, as are those with more wealth and those with military experience.