There were 6 DB5's used in the making of Goldfinger. 2 were unfortunatly written off during filming, one belongs to the Bond museum, and Aston has one. As for number 5 and 6, I don't know. Maybe there is a couple with a delapidated DB5 in there garage.
I do know that the reason for the multiple cars is that all of the gadgets that appeared on the film car, couldnt be fitted into one single car, so there was one where the ejector seat and bullet shield worked, one where the machine guns worked, etc etc. The two that were written of had no gadgets at all, and Aston gave them to the film with the purpose of crashing them! How depressing!
And an easy way of telling a DB5 from a 6 is the rear side winow. On the 5 it the top edge of the window follows the curve of the roofline, but on the 6, it bends sharply away at the back. Much like that of the new Astons.