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?

One is plenty enough. In OTB tourneys, players are pretty well aware of the rules, litigation is rare and most of the time, easy to resolve. Appeals are well stipulated in the rules and regulations of every serious tourney.

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

Im sure that most players who are present are pretty well aware of the rules, but since this particular tournament will be taking place at a university, this will likely be the first tournament for many players which might pose a bit of a challenge at least in the beginning rounds.

Ouch. I'd then recommend one certified main arbiter, and some (2-4) expirienced players to help out both with rulings as well as taking in results, inputting them into a computer, and pairings. And of course, a short clarification of rules before the first round is a must in this case,

right, my mistake. this will not be a FIDE or USCF rated tournament (although the standard rapidplay (20min) rules will apply). as for it being rated at all, I believe the particular swiss system program gives an approximate elo range upon the end of the last round.

I think Puchiko has the right ballpark. though to respond to brfc, i may tuck away 10.2 after just reading it :P. thank you everyone for your input