How many times?

So here is a question. How many times do you actually push space after each sentence. Are you supposed to space twice after each sentence or just once, or does it even really matter? I thought about this because someone in my previous English class said I needed to have a second space after the sentences, I never did it but I think it is time to find out. HERE the opinion of the writer is that only one space should be used. HERE, the writer says there are exceptions to the rule but for the most part just use one. Reason: two spaces were used in the days of Typewriters so that readers could distinguish between two sentences more easily.