Are Dorking chickens rare?

They are not as popular as they once were but they are not rare everywhere.
They are not used as mass stock in corporate farms but Dorkings are still popular in certain areas with small farm breeders and readily found in home flocks. The white skin of the Dorking has made them unpopular in America as meat birds and their need for unconfined spaces make them unsuitable for battery farm use.

Another answer

I guess that would depend on your definition of rare.

The American Livestock Breeds Conservancy www.albc-usa.org lists the Dorking as "threatened", meaning that there are less than 1000 breeding birds in the U.S.

Rare Breeds Canada www.rarebreedscanada.ca states on there website that they are also on the critical list.

Rare Breed Conservation Society of New Zealand www.rarebreeds.co.nz/chooks.html lists the dorking as endangered

The Rare Breeds Trust of Australia www.rbta.org/silver.htm also considers the Dorking as a rare breed and in danger of being lost forever.

Moonbeam poultry farm in the UK www.moonbeamsland.co.uk also considers the dorking a rare breed

Much of the reason these birds become endangered is because people do not understand the importance of the situation. Sometimes people see someone with a couple and assume that ,because the guy down the road has a couple, they couldn't possibly rare. The fact is, if you consider them rare of not, these birds will most likely not be around in 50 years without help.