one space only

As a working professional you want to make the best impression and that comes down to details. 

What's the surest way to reveal your age online? By putting two spaces after punctuation when you type. Two spaces after punctuation was the rule back when we typed on typewriters. This was because typewriters used a single, monospaced font. In a monospaced font, all letters take up the same amount of space. An "i" takes up the same amount of space as a "w." To make reading easier, the two-space rule was born to give your eye a break between sentences. Computers negate this problem because they automatically space fonts proportionally. Thus, you only need one space after punctuation. There are a couple of exceptions—the fonts Courier and Monaco are still monospaced. Don't use them. Switch to more widely recognized fonts like Times New Roman or Arial, and get into the modern habit of using only one space after punctuation. It will keep your communication from looking like you wrote it on a typewriter.