You can create a list of fade-ins and fade-outs, like this (supposing the numbers are sequences):
var fade_in_list = []
var fade_out_list = []
if num == 1:
fade_in_list = range(2)
fade_out_list = range(2, 11)
if num == 2:
fade_in_list = range(5)
fade_out_list = range(5, 11)
# and so on for each num...
# then just iterate on each list to fadeIn/Out each list element
for i in fade_out_list:
music_a._fadeOut(i)
for i in fade_in_list:
music_a._fadeIn(i)
See range() function for more info.
Also, if they are not sequences, you can create list indices on each condition, like:
if num == 18: # arbitrary example
fade_in_list = [1,5,6,7,8,9,10,12]
fade_out_list = [2,3,4,11]
Or better (if I understood your code correctly):
func play_arrangement(num):
for i in range(num + 1):
music_a._fadeIn(i)
for i in range(num + 1, 13):
music_a._fadeOut(i)
