Two. That's what I learned in school, back --baaaaaaaaaaack-- in the days of typing class.
Two spaces were supposed to improve the reader's chance of appreciating the end of sentence condition. Now I still rather dislike software that removes extra spaces I put between sentences.
As for question marks or other punctuation, there's no space between the word and the mark, at least for American writers. I always thought an exclamation point could use that space so sometimes I provide it. Let the recipients of my posts or mails sue me if they don't like it !!! 🙄 and the more exclamation points, the merrier. 🙂