Our auction is for a small school in a small place - we'd like to raise at least $12,000 (my goal is $15,000). We have a group of people requesting item donations and then 2 people who are arranging the logistics. We called a meeting for the auction and drafted everyone who showed up.
I'm coordinating the donations committee. I had contacted national companies and built the donations spreadsheet prior to the first meeting, and everyone who came to the meeting and was willing to drop off letters at their favorite businesses gave me their email addresses. Most of our communication is done via email, unless the logistics people want opinions. Then we all meet again and go over where we are for both groups.
For a larger school or a more elaborate evening, you may need more people involved. If so, then I could see the need for committees for publicity, venue/food/entertainment, donations, and auction procedures.
I hope you get additional information from others, but our experience has been that a smaller group who has time to take on the auction can handle the load as well as a larger group with more defined roles. Adding more people does give more ownership to more people and increase your attendance, as long as you can verify that tasks occur in a timely fashion.