I dont know if anyone can give you a production number, but nickel usually is a small percent of most runs. Those guns are gorgeous and I'd grab it.
My 586 ND was stamped to be nickel and they changed their minds!
I do not know of published numbers of nickel plated S&W's, but it is generally accepted that roughly 10% of carbon steel S&W handguns came from the factory with a nickel finish.