Yep, one belt.
If you have a single belt alternator and water pump pulley, then all you need is the crank pulley and belt.
If you have a two belt water pump pulley and want a single, order one for bout any A body 318 car without AC.
what i have now is a crank pulley with two tracks and my alternator and fan both have one, so the line up is thus, alternator to crank, then my water pump to crank, so if i was to change out the alternator pulley to a dual track they would all line up and i could then run one, only question is, is that proper? and is there even a belt for that or would i just have to measure to get as close as possible and use the alternator to take up slack?