Jules Richardson wrote about the Twiggy Mac:
How do they know it's the only one?
They don't. But in the 29 years since they were made, there's no
indication that a complete unit has ever turned up for sale before, nor
anyone publicly stating that they have one. It's unclear how many of
these were made, but probably not too many, and of the few that ever
left Apple's hands, most were probably returned to Apple and either
modified to use the 3.5-inch drive, or destroyed.
It is certainly far more rare than an Apple 1. If there are any others
left out there, they're well-hidden.