Dennis,
 
I don't know exactly what kind of exhibit you are talking about, in fact
that is the first thing to do --- decide on what you want to show; a
family, an area, a variety, a color, marine, land, etc...  The list goes
on.
 
A friend recently set up a generalized exhibit at a local library.  He
picked shells he thought would interest the general public (Thatcheria
mirabilis, Pecten pallium, Conus textile, Oliva porphyria, Murex
brandaris, a freshwater mussel and others that I do not remember,
including land shells) and not those that were rare or necessarily
unique.  Each shell had a data card giving the name, locale, and an
interesting fact or two about the shell or the family of shells.  Thus he
had a blurb about the dye from the M. brandaris, the buttons and cultured
pearl centers from the freshwater mussel, and the scallop industry on the
P. pallium.  The shells were NOT crowded (although as a collector some of
my fondest memories were displays with shells almost piled on top of each
other).  As an example for his disply, a display case with a 20 inch by
30 inch shelf may have had only 10 or so shells.  This left room for a
3x5 size card or more with the shell facts next to each shell.  It was a
very popular display.
 
I think you can do this about anyway you want - after all the stars are
the shells and they just about always look good.  Good luck
 
Tom Eichhorst