I put as many as will fit. I also put as few staples as possible. Never in 30+ years been tagged on a staple violation. If an inspector is looking at staples, he isn't looking at the real issues.
I had a guy gig me for two cables side-by-side down a 2x4. He said I couldn't do that. I said I have plenty of room and am easily 1-1/4" from the edge. He almost didn't let it go.
That guy was a jerk. One of those that HAVE to find something.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.