Usually i use one. I think two is a big one. There should be only one. I think it does not depends on personnel style. There is some rule for this but i really don't know.
I'm only twenty-one (well about to turn twenty-two in december) and I was always taught to use double spacing after the period in school. To me, the double just looks better, and it would seem that it would be easier to read that way, so most editor and publishers would probably prefer that way. Though I can't say for certain.
