How often do you email a supervisor?
When you email him, at your current rate, how often do you get responses? Has he indicated that it bothers him? (either directly or by showing signs that it annoys him) If not, why are you worried about it?

I'll add that the right question to ask is probably not “how often can I email him?”, but rather “am I emailing him with the right questions?”. What will definitely annoy your advisor, and pretty much any other person, is if you ask them series of questions to which you could have found the answer yourself, or questions whom they are not the right person to ask.