The United States of America
USA has had 50 states since 1959.
The District of Columbia is a federal district, not a state. Many lists include DC and Puerto Rico, which makes for 52 "states and other jurisdictions". Maybe that's the root of the myth? Or do people think of a deck of cards, the weeks in a year, or the country code of Mexico? Does the number 50 appear to be too rounded to be accurate?
The flag has 50 stars, one for each state. It is updated as needed.
A common explanation is that the flag doesn't include Alaska and Hawaii, thus 50 + 2 = 52. But Alaska became the 49th state on the 3rd of January 1959, and the flag got its 49th star on the 4th of July the same year. Hawaii became the 50th state on the 21st of August the same year, and the flag got its 50th star on the 4th of July 1960. Here's some older glories:
Counting stars might sound like a pastime for really bored nerds, but since the stars are arranged in certain patterns, some of which are quite distinctive, a small piece is often enough to identify the version of the flag.
The straight column of stars tells us that this has to be the 48-star flag. (On a genuine photo.)
In this flag there are six rows of stars, with the 3rd and 5th being longer than the 2nd and 4th - this doesn't match any of the flags above. That is what one would expect from a historically accurate painting, since Teddy's fleet sailed in 1907. See the links below for this version of the flag, and many more.
I presume, however, that anachronistic flags are very common indeed. The next time I see a World War movie, I'll keep an eye on the american flags - they should feature the same simple 6 x 8-pattern as the one on Iwo Jima. (Not that I would mind if they don't; I'm not that bored.)
Thanks to Christer who came up with this factoid