That's hard to say, Bee.
The official definition of minimum altitude enough to cause altitude illnesses is 8,000 ft.
Some other factors for AMS are; state of acclimatization to begin with, time ascending, workload getting there, activity while there, how long a stay, etc.
This is not your normal hiking scenario, but if an
unacclimatized person takes a helicopter from sea level and stays at:
*8,000, then I doubt 100% of people get AMS.
*to Whitney 14,500? Still not 100%, although I bet very close.
*18,000 ft (the height of Kalapattar above Everest Basecamp)- I am told that you would pass out after 30 minutes, maybe even before you felt AMS!
There are studies from various peaks like Whitney, Rainier, reporting the incidence of AMS. Not sure about a height where 100% are affected, but it is above that. Here is the link to Ken's study on Whitney
http://www.ncbi.nlm.nih.gov/pubmed/18799993
