Answer :

It really depends on how precise you're going to get.
You could get away with only two parts, but to be really precise, you should probably divide it into twelve (12) parts, because 5/12 means five out of twelve parts, so its would be right on the fifth subdividing marker line. 
You should divide it into 12 parts because the denominator(the number on the bottom) is 12. Then, you should shade or circle 5 out of the 12 pieces.  Remember that 12/12 is 1 whole. 

Hope i helped!