Question

why one year have only 12 months ?

Answer 1

THE LENGTH OF A YEAR IS DETERMINED BY HOW LONG IT TAKES FOR THE  EARTH TO COMPLETE IT'S ORBIT AROUND THE SUN (365  1/4 DAYS) THAT'S WHY HAVE ONLY 12 MONTHS
    
                              PLEASE MARK AS BRAINLIEST

Answer 2

A year has 365.25 days.(because the earth completes 365.25 rotations while completing one revolution around the sun.)
now these 365 days were divided into 12 months because there are 12 lunar cycles in a year. Also, by doing so, addition of one day in the month of feb for a leap year became easier.
Also 12 months can be easily grouped to sync with the seasons.