This is a difficult question to answer because Nora Roberts' writing schedule can vary significantly from year to year. However, based on interviews and other public statements Roberts has made, it seems safe to say that she usually writes at least one new novel every year. In some years, she has published two or more new titles.
In an interview with The Guardian in 2014, Roberts said that she typically spends six to eight hours writing per day, and that she usually finishes a first draft in about three months. This means she could theoretically produce four or more first drafts per year. However, it's worth noting that Roberts often takes a break from writing after finishing a first draft, and she also revised some of her older works during this time. So the number of new books she actually publishes in a given year can vary.
Here is a link to the Guardian interview: https://www.theguardian.com/books/2014/jan/11/nora-roberts-interview-jd-robb-writing
