I reckon there were loads more than 500 of these made. They were manufactured from 1992 to 1995 - if Ford only sold 150ish a year, i.e. 12ish a month they would have been immediately axed as a complete flop. If you think about it Ford sold the complete stock of RS500s in a matter of months.

If you have a quick look on the web you'll find loads of them for sale - which also gives the impression that they are not as rare as people say.

An almost identical car was sold on the continent, but it was called an XR2i 130, and not an RS1800

The DVLA may be able to give you an idea of how many were registered if you pay for a query, but you'd have to ask about Fiesta "RS1800", "RS 1800" and "RS" and add them all together. I believe there is also some way to get Microcat to report on model numbers - but I don't know exactly how to do it...