how did you figure out how much inventory to have with you for a
The short answer is "as much as possible".
The longer answer is, enough to fill your display and refill.
Practically speaking, this really means set up your display so that
no more than two-thirds of your work on hand goes in initially so it
won't look bad as you sell pieces.
As a side note, one mistake I tend to fall into, as do many others,
is overfilling cases. One artist described this as looking as though
you have put out everything you ever made. I always feel as though
the piece I didn't put out yet is the one that person would have
bought, but I am also convinced that a somewhat sparing display
looks far better than a crowded jumble.
So if you have relatively few pieces, you can put your display
across the 10 foot space, at the front. If you have a lot, you can
do an L or a Z into the interior space to accomodate more cases.