I don't have any data to back this up, but I think they sold hundreds of millions of high end (and expensive for the time) smartphones running Symbian. Remember the N line? The E line? Communicators? All running Symbian, all bestsellers for years.
Edit:
As you write they were expensive. So while Nokia was the undisputd leader in the segment for a couple of years the absolute numbers were not that big. I remember some year the worldwide phone market was 400 million a year. Nokia had over 100 million of them, but smart phones (Symbian) only single digit millions.