Harlequin Mills & Boon published 135 novels by Betty Neels, with copyright dates (in my editions) from 1969 to 2001 inclusive. That’s an average of 4.1 books copyrighted per year, or one every three months. Her highest-volume year was 1975, with six copyrights; the lowest was 2000, with just two copyrights. In that 33 year span, there were nine years (27%) in which she copyrighted three books, ten (30%) with four books, and 12 years, 36% of her copyright-ing career, in which she copyrighted five books.
My land.See graphic below, and feel free to pose questions and corrections in the comments section.