In today’s day and age, one space. Primarily because most of our writing today is done on the computer. Word processors do just that: they process our prose by auto-formatting it. Part of how a word processor “reads” your document so that it knows how to format it is by analyzing your “white space”. You confuse it when you double up on spaces after sentences and returns after paragraphs because of arcane typing rules you learned in high school on a Selectric back when LBJ was in the White House.

One space. Thank you very much.