Certain lists are easily quantified, as it is unlikely that 7 continents or 8+1 reindeer pulling Santa's sleigh will change by even 1 in the foreseeable future. Speaking of seasonal songs and poems, there will always be 12 Days of Christmas and their corresponding gifts. The number of state capitals in the U.S. appears to be quite stable. Other categories, though, are constantly in flux. As of now there are 116 elements in the periodic table, but this is subject to further experimentation and discovery. The number of U.S. presidents will constantly increase. The number of oceans on our planet varies from 1 to 7, depending on the definition of "ocean" that one chooses to use. Some lists never did exist, with the famous "7 Seas" coming to mind. There are numerous seas, gulfs, and oceans which could have been included at various historic times, and the saying really just exists as a cliché rather than an actual accounting of specific bodies of water.
Some people enjoy just knowing the NUMBER of items on a list. For instance, there are probably few who could say off the top of their head that there are 3086 counties (or equivalent) in the United States. A handful more might know that Texas has the most, with 254. Musicians enjoy this sort of thing with "How many symphonies did ____ write?" Most people know that Brahms wrote 4, Beethoven wrote 9, Mozart wrote 41, and Haydn cranked out a whopping 104. There is, however, something more alluring in actual names than just numbers for many.
Shorter lists are, of course, easier to remember than long ones. Most people can name the 9 planets (or 8 if you classify Pluto as the largest of the Kuiper Belt objects) that orbit our sun. Few can name the 47 prefectures in Japan, especially if they are not Japanese. There are lists whose exact contents almost never come up in everyday conversation. While the 7 deadly sins are relatively famous, the 7 contrary virtues are rarely mentioned. For that matter, there are also 4 cardinal virtues (definitely obscure) and 3 theological virtues (more popular) that make up the heavenly virtues, not to be confused with the contrary virtues. Depending on your cultural heritage, it may be easy or difficult to name the Noble Eightfold Path of Buddhism or the 6 articles of faith of Islam.
So, as you read this, did you test your memory on at least a few of the lists? Tempting, isn't it? There is something very captivating about these little puzzles and, while most people wouldn't sit down and try to name the 3086 counties in the United States, almost nobody can resist having a try at the 7 dwarfs.
For the curious: