LOL I have these memorized from Bo Friberg's table: (oh all those brain cells going to waste storing data like that)
NOTE: based on rounds height like a wedding cake about 4" tall.
16 inch 72-84
Those figures are for wedding cake, so I would say you could go up from there. Also is it after a many course big dinner, or is it just a cake and coffee event... this will influence amount. I will add there is a quality factor too, if the cake is really delicious more seems to be needed.;)
Also, those figures above are based on cutting like a pro, cutting an inner round, then wedges from the outer circle and wedges from the inner circle.
I haven't seen the Wilton chart, but if you go with a 12x18 from their chart, check that your height is the same as what they're referring to.
Also, you could send specs as to how to cut for min. 50pcs if you're not involved with cutting.
BTW If you use a large pan you can consider using those insulators around the edge for a more even rise.