Well, usually the machines that can do a one shirt minimum are digital. And most digital shirt printers can't print a large enough area to be considered "all over" but most can get a pretty big print.
A digital print on a Brother
DTG (Direct to Garment) printer can print 14x16 area. There are others out there that can print larger I think - I have seen them at tradeshows, but don't know them by name.
Other than that, I think a 50 pc. minimum on large print is pretty good. And these 50 will probably be screenprinted & not digital. You could also call some local screen printers and see what their maximum print area is, you may find one that will do an 18x20 area with only 12 pc. minimum.