There are 16 possible routes you can take in total

A means take a left turn, B means take a right turn. For example B-A-B-A means take a right to Resort Islands, left to Villa Rica, right to Monument Valley and left to Metropolis.

#1 = A-A-A-A
#2 = B-A-A-A
#3 = A-B-A-A
#4 = B-B-A-A
#5 = A-A-B-A
#6 = B-A-B-A
#7 = A-B-B-A
#8 = B-B-B-A
#9 = A-A-A-B
#10 = B-A-A-B
#11 = A-B-A-B
#12 = B-B-A-B
#13 = A-A-B-B
#14 = B-A-B-B
#15 = A-B-B-B
#16 = B-B-B-B

You can see your progress of finished routes in the save game screen (counting the dots from left to right).