Well, a glow plug typically draws about 3 amps or so. So 3 amps times 9 cylinders is 27 amperes of current. That is a pretty hefty amount to run to the engine all at once.
Unless you use a 9 cylinder glow starter unit, you want to have each glow ignitor separate from each other (albeit they all have a common ground though). You can't just run them all in parallel off of one battery because if a glow plug or two or more burns out and is open then the other glow plugs may wind up taking the load and getting too much current then.
So the classic 2 D cell in parallel battery pack per glow plug is a good starting place to keep the costs down.
of course trying to keep all the wires organized and neat gets to be a problem.