Here's a list of how custom music is going to play if the "Custom Music Advanced (1-50)" pack option is used. the rest should pretty much be self-explanatory.

Prioritized = Higher chance of being played.

Menu:
custom1 - prioritized
custom2 - prioritized
custom3
custom4
custom5
custom6
custom7
custom8
custom9
custom10

Creative:
custom11 - prioritized
custom12 - prioritized
custom13
custom14
custom15
custom16
custom17
custom18
custom19
custom20

Game:
custom21 - prioritized
custom22 - prioritized
custom23
custom24
custom25
custom26
custom27
custom28
custom29
custom30

Nether:
custom31 - prioritized
custom32 - prioritized
custom33
custom34
custom35
custom36
custom37
custom38
custom39
custom40

End:
custom41 - prioritized
custom42 - prioritized
custom43
custom44
custom45
custom46
custom47
custom48
custom49
custom50