Only ever one.
The second is completely superfluous in the modern day. Professional typographers agree. The best practice has been one space for over 100 years. What caused the two space practice to creep back in was, of course, typewriters and early computers/printers. Monospaced lettering needed an extra space to improve readability.
Now it is never needed and should never be used. But unfortunately there's a large portion of people who have been taught that double spaces are correct and sadly there are people who still assert it as best practice. But it isn't.