To contradict what David said (And I could be wrong). In my experience, it's fresh instances you enter per hour - not how many times you reset (Although when you reset it's a fresh instance - so take that into account). SM seems to count as 1 instance total for this purpose.

I've had times where one of my guys accidentally logged out in an instance (My tank booster), and the other 4 didn't. When that guy logged back in to a instance it then counted it as one fresh instance. How did I know? Because this was the only guy locked out of the final run for the hour. And we all entered at the same time.

Also, I noticed loot and reset started a new timer when I went from SM -> ZF, even though I didn't reset the instance. If didn't restart the timer from SM-SM