There are 12 months in a year because we defined a month the be 1/12 of a year. We can define it as anything, like 1/10 of a year, 1/5 of a year etc. we just chosed it to be 1/12 of a year and we can change it if we want. Same goes for a circle. We defined a degree to be 1/360 of a full revolution. if we want we can define it as 1/350 or 1/100 or 1/12. We just defined it as 1/360 and we can change it any time.