Two.
Not one.
Two.
Apparently there has been a change, years ago, that state only a single space is required instead of two. Searching online can bring up a variety of articles written on the subject. I won't bore you with the details.
When it comes to writing--and this goes for much of creative, fiction writing--as long as you can draw the reader into your story, it doesn't matter how many spaces you use after a period. When I asked people that I know who read a lot on this subject, they shrug and say they don't even pay attention to it. All they want is the story.
If you're writing a paper and your college professor requires you to put one space, then do that. No sense arguing. Just do it. But if you're writing a story, write it so well-crafted that the reader doesn't even notice. For me, it's habit to use two spaces.
And I will continue to do so.