As an arbiter myself, the one arbiter would be under a ton of pressure to manage 100 people. 2 would be adequate for a congress, and for a rapidplay you'd need 2 good arbiters for them to cope with taking in results and pairing in that short time. Trust me, doing a 70 man rapidplay on your own is a nightmare!!! I've been through it.
In summary, if congress, if you can really only have 1 arbiter, then perhaps you could get away with it. But I would strongly suggest 2.
Also, the players don't know the rules as well as you think! The amount of times people have promoted a pawn incorrectly is amazing, and how many people ask for a 10.2 claim. Makes me shiver inside xD
How many arbiters are suggested for a 100 player chess tournament? Of course the more the better, but what is the minimum that is normally needed to allow the event to run smoothly?