Roughly one item for every two to three guests. More than that spreads bids too thin and leaves items with one bid or none.
A fraction of the item's estimated value. Lower entry points bring in more bidders, and more bidders usually means higher final prices. The guide includes a formula you can apply to every item.
Five to ten days is typical for an online-only auction. For an in-person event, bidding usually runs from doors open until shortly before the formal program ends.