My question is: how much inventory should a person have on hand?
Can't give you a single universal number, as it depends on the type
of the work, how much space you have to show in, how you display it.
But of course, if you have more work, you'll be likely to sell more.
Your guess sounds like a decent start for many people. You can't sell
what you don't have.
After that, I'd use as a guide the look of your display. How will it
compare to your fellow exhibitors? If there's too little, or the work
too simplistic, you may find viewers not taking serious time to look
or taking you seriously.
Conversely, too much on display crammed into a small place might
seem impressive to you, but viewers eyes might be just overloaded,
and they'll just keep moving.
Your display needs to catch the viewers interest and attention, and
then show them enough from which to choose, and from which to see the
full depth of what you do. If you've optimized that, extra inventory
on hand but not all on display can be good if things are selling
well. The price points you're setting will also have an impact on
this. If all your work is priced in the thousands, you may not need
as many pieces, as sales likely will be slower in volume (you'll also
have to pay more attention to security in this case).