Apparently the answer is ONE. I can already feel myself wanting to use two spaces after each. sentence. I. write!
According to an article on The Atlantic, the only reason that we ever used two spaces after the end of a sentence was because of a flaw in typewriters, and now every major style guide recommends just one space. Why did typewriters lead us to use two?
Well, with typewriters, all letters, numbers, and symbols all occupied the same amount of space (called "monospaced type"). So an "i" would take up the same amount of space on a line as a "W", thus leaving much more white space on the page. With more white space, it was harder to detect when a sentence ended, so the standard became to use two spaces.
Here's a great example from The Skilled Workman (which has a ton of typography tips for publishing):
Now, and since the 1970s, besides the little-used "courier" font, we use "proportional fonts" where the letters only take up the space they need. As a result, it's much easier to see where sentences end and begin. So the extra space has been dubbed "unnecessary".
To read the entire explanation from The Atlantic, click here. And for an interesting counter-argument, click here. :)
Typewriters didn't only affect our sentence-spacing, but the beginning (and arguably modern misuse) of the "Caps Lock" key. Check out a great article about that, too, on Slate.com: Click here!