I've answered this question before...and will again, I'm sure.
There is more than one way to answer this. You can easily assume that half are gone. Wrecked, trashed, abandoned, stolen, burned, etc...
That leaves about 2,000.
But...you can go on the compliance of the '68 Shelbys in the SAAC Registry. It's about 40%. If you went on that same percentage...you'd get 1,647. But, not everyone registers their GT/CS. I have just under 900 cars. If you double that--you're at 1,800. But...half are gone. So...I really think that the number left is closer to 1,800 as a very educated guess.
Then you take the numbers of those with engines, colors, etc...and you REALLY get a feel for rarity. Then you suddenly get a real feel for just how rare your GT/CS is....and appreciate it even more.
But no one will really know. There is no way to track cars out there....and there are those unregistered (state DMV), in barns, garages, etc..
And there is the clone issue that complicates things, too.
Paul N.