It all depends but 8-12 is typical depending on the program. These should be your most exciting desirable items and the order of your auction items should follow a bell curve starting with lower price items and moving up to highest value and then back down ending again with lower price point items giving guests who missed bidding in the start to still have the opportunity to win.
For the extra items from your live auction, create a special area called the "super silent" or "Premier Table" and move the excessive items from your live to this table. Then have your Auctioneer/MC highlight some of the items from this table to drive traffic through your silent auction as well.
Our group is a small Catholic school and we are on our 3rd year of our Annual Gala event. The past 2 years we have heard some complaints of our Live Auction portion taking too long. We were wondering what is a good number of items to have for our Live Auction portion of the night?