Why do years have 12 months?

There are twelve months in a year because 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).

The moon orbits around the earth about every 29 days - that's a lunar month. If you divide 365 1/4 days by 29 days, you get about 12 1/2 lunar months in a year. So to have an even number of months in the year, days were added to some of the 12 months to get us to the 365 days (366 in leap years) so that we would have an even 12 months (calendar months) in a year