It is common knowledge that many Christians are superstitious about the number 666. This number is called the mark of the beast in the book of Revelation chapter 13. Some people think it is a sign of the devil or something evil whenever they see this number. However, this belief only applies specifically to Christians who believe in the significance of the bible in the first place. Nobody of any other belief has reason to care about this number.
Some people believe other numbers are unlucky such as the number 13. There are infinite numbers and tons of people who have beliefs about them.
As someone who likes mathematics and numbers, I have no tolerance for anyone who makes a big deal out of a specific number as having meanings which have nothing to do with numbers representing quantities of objects which can be added, subtracted, multiplied, or divided.
Here are a few links about the number 666 for anyone who wants to be entertained or annoyed by what people believe about this number.
There are real problems in the world to be concerned about. Those who fall apart because they buy something at a store and their total is $6.66 or their credit card bill for the month is $666.66 are just stressing themselves and others out for no good reason.