It’s traditional to type two spaces after the period (full stop), but nowadays, because of electronic publishing, some style manuals tell us to use just one space. You’re not wrong if you do either one, but just don’t use three spaces.
That’s a very informative article. I should also mention that it’s not advisable to use two spaces after a period if you are typesetting, because the wider space on several lines can result in what are called “rivers”. Those are the illusion of white tracks running down the page, and it’s caused by the accidental aligning of these double spaces.