So far it's 43 books in 25 years.
One book at a time... though I'm usually doing the research for others while I'm writing, but that sort of research is fairly desultory and I like to stick to the book being written - and writing a book concentrates the mind so the research is more productive.
Of course some days are easier than others, but my worst day is better than being in most humdrum occupations.
Looking back, of course, it was irresponsible, mad, forlorn, idiotic, but if you don't take chances then you'll never have a winning hand, and I've no regrets.
Judy couldn't move to Britain for family reasons, so I had to come to the States, and the U.S. government wouldn't give me a Green Card, so I airily told her I'd write a book.
Writing is a solitary occupation.