chicago manual of style says one space.
good reference point. looking up the other biggie for grammar style, the MLA, this is what they have to say:
"Publications in the United States today usually have the same spacing after a punctuation mark as between words on the same line. Since word processors make available the same fonts used by typesetters for printed works, many writers, influenced by the look of typeset publications, now leave only one space after a concluding punctuation mark. In addition, most publishers' guidelines for preparing electronic manuscripts ask authors to type only the spaces that are to appear in print.
Because it is increasingly common for papers and manuscripts to be prepared with a single space after all punctuation marks, this spacing is shown in the examples in the MLA Handbook and the MLA Style Manual. As a practical matter, however, there is nothing wrong with using two spaces after concluding punctuation marks unless an instructor or editor requests that you do otherwise."
so either is ok, but they are already choosing to use single space as the primary example and it is trending as the standard style and will only continue to grow.
soon enough, single space will be the de facto standard.