Seven 

There are seven major keys on a piano. There are seven major colors in a rainbow (red, orange, yellow, green, blue, indigo, and purple). There are seven days in a week.

There are different ways in classifications. In music, there are different numbers of keys in different cultures. Nonetheless, seven major key is the most prevalent way. There may be different numbers of major colors in a rainbow in different languages. But at least in Chinese, seven color (七彩）is the standard way to represent the full color spectrum as well. Similarly, there are different ways to group days. In the place where I grew up, the market in a nearby town opens every ten days. Still, seven day a week is the predominant cycle in the world.

Is there anything special about seven? Seven is a prime number. Seven is the only prime number between six and ten. Compare with other frequencies. There are three prime numbers between one and five, two prime numbers between eleven and fifteen, and between sixteen and twenty. Seven is quite unique.

As for finer tones, there are twelve keys (seven majors plus five minors) on a piano. A pack of crayons usually contains twelve colors. A dozen is twelve.