Maximum number of targets doesn't affect. I keep it default as 1.
check targets allowed as ground,structure,debris,air,item,ward
attack type as missile
attack range & acquisition range
For the spell Barrage, see the attachment.
Another option is using Moon Glaive for multiple attacks, probably you have seen it in some Tower Defense map. Set Techtree-requirement NONE for Moon Glaive.
Hope these help. Very easy stuffs.