Well, hand built, you mean? But more than 50 of each made, right? And Koenigseggs for example come from a country with fewer people than Ohio, so to be a supercar from Sweden shouldn't take that much production to get you recognition. I see they marked their 100th car Dec 2nd 2012, so probably 130+ by now. 139 Zondas alone had been built by Pagani as of 2009. Something near 100 McLaren F1's as well. Plus McLaren has gone on to become a major manufacturer.
14 Moslers, only car from this company, and some of those cars were made in England... so no
Venom GT, 29 cars, from a tuner, doesn't quite cut it
78 Saleen S7's, and the only car ever made by a company known for tuning, not manufacturing, but I'd partially include this in the list of American supercars because more than 50 were made
SSC Aero can't find the production figures, but coming from Shelby and continuing with the Tuatara, yeah, this too should probably be included
But I'd still say the only American supercars that don't get a asterisk for low volume are the Viper, ZR-1, and Ford GT. But that's just me and we can agree to disagree, for me there need to be enough of them made that people actually have a good chance of seeing one without being from LA, Miami, or Houston.