The N refers to Special Forces and the 7 refers to the highest level of proficiency.
N7, to me, simply means the best of the best among humanities soldiers. Soldiers of incredible skill, ready for whenever the next challenge presents itself, and are prepared for whatever that challenge may be. They still adhere to the chain of command. They still answer to their superiors, but are far more respected than the common soldier.
They are the equivalent to that of the Turian Blackwatch, and any other team among other races that are the most exceptionally skilled and are sent on only the most important and dangerous missions.
(Saren was supposedly a member of the Turian Blackwatch before becoming a Spectre to put things in perspective)
The only soldiers that are better in skill and proficiency than N7 (or a respective team such as I mentioned above) are the Spectres, but that's simply because they have no rules to govern them. No laws that they need worry themselves with. The mission is the only thing that matters.
And we did see other N7 operatives. Lee Riley, who led the second team at the fuel reactor on Cyone. Kai Leng (was an N7 before being honourably discharged). David Anderson is N7 (one of the first to receive that honour).