Will sneak experience increase more quickly if 2 people can discover me instead of just one? If so, how much more quickly? And again, if so, is there a limit, for example, will having 20 people be able to discover me increase it even more quickly than 2 people?
Not really answer to your question, but still:
Quickest way to level up sneak. Go to any tavern and find the barman or barmaid. Crouch into a sneak. Find a corner behind the barman and walk backwards while still crouched. Continue walking backwards as the sneak progress bar climbs progressively higher. You can even leave a rubber band on your controllers thumbstick to keep the progress climbing while you step away for an hour. 1 hour of this should increase your sneak at least 30-40 levels minimum. The taverns and inns are the best place to do this as there's always at least 2-3 people within distance of you and if you stand behind the barman most will not have line of sight making it next to impossible to fail the skill check.
So the more people are in the area you are practicing sneaking-the more chances you have to really practice it.
Don't forget to trek out to the thief stone outside of Riverwood first and get that on you. 20% increase to the speed of 'stealth skills' http://www.uesp.net/wiki/Skyrim:Standing_Stone