Twenty-five to forty items for an hour. That is the range that works, and the reason is pacing rather than how much you have.
Why that range
An item that is moving takes 60 to 120 seconds: hold it up, say what it is, name the flaw, let the bidding run, close it. Forty items in an hour is 90 seconds each, which is the rhythm a room stays awake for.
Fewer than fifteen in an hour and the gaps between items get long enough for people to leave. A room empties in the dead air between one sale and the next, not during an item.
More than fifty and you are rushing things nobody saw properly. That costs you twice: lower prices now, and disputes later from buyers who did not actually see what they bought.
What matters more than the number
That you do not run out. A show that finishes its lineup in twenty minutes and limps on is worse than a shorter show. Queue more than you plan to sell and end while people still want more.
That the good things are spread out. Not all at the start — people leave after them. Not all at the end — people leave before them. Something worth staying for every ten minutes.
That the first item is strong and starts immediately. Most of the people who leave a show leave in the first minute, and they leave because nothing is happening. How to go live.
For a first show
Fifteen to twenty. Deliberately fewer, because you will be slower than you think, and because finishing confidently beats grinding through forty while losing the room.
If you have far more stock than that
Run more shows rather than longer ones. Two hours of forty items each, on two evenings, beats eighty items in one three-hour sitting — for you and for the room.
The leftovers are not wasted: unsold items go into the next show, which is a large part of how a lineup gets built. What happens to an item nobody bids on.
If you have far less
Run it anyway, with twelve. A thin show that happens is worth more than a cancelled one, and cancelling on people who set a reminder is the expensive move. Can I cancel a scheduled show?
The practical preparation
Queue the lineup before you go live — Prepare lineup the day before. Going live and then building the queue is admin performed in front of an audience.
More: how long should a show be, how to put an item up during a show, how to schedule a show.