I can't be sure what you would use it for but I do know that it can be used for converting kilometres into miles i.e. 5 miles =8 kilometres and so on and that many flowers have a Fibonacci number of petals.
Fibonacci's accomplishments were the creation of the Fibonacci number sequence and he also was responsible for the spreading of the Hindu-Arabic numbering system.
the first seven Fibonacci numbers are 1,1,2,3,5,8,13. 13 is a Fibonacci number.
The 365th Fibonacci number is 8531073606282249384383143963212896619394786170594625964346924608389878465365.
The 100th Fibonacci number is 354,224,848,179,261,915,075.
123 is not a Fibonacci number.
The twelfth Fibonacci number is 144.
The 22nd Fibonacci number is 17,711
How to answer with formula
Yes - 5 is the fifth Fibonacci number.
Leonardo Fibonacci was an Italian mathematician who advocated the use of Arabic numerals. He wrote a book called LiberAbaci, which demonstrated the use of the number system. Fibonacci numbers, a sequence used in the book to illustrate its message, had been known before his time, but the numbers took his name because they were connected with the book.
Fibonacci is important because he introduced the Arabic number system to Europe. His sequence is much less important - it is mainly a mathematical curiosity.
Decimal numbers were in use in Europe well before the time of Fibonacci so he would have "related" to them when he started to count!