The strand count varies -- the more stands, the thinner the individual strand. The key to being 14ga is the total cross sectional area of the copper strands, and those are very close no matter what the strand count. Those with higher strand counts are more flexible and easier to route though.
Some will argue the "skin effect," which means that much of the current flows near the surface of stranded wire, but that is negligible as far as I have ever read.