People have asked whether it is possible to write 10 or more cookbooks annually
Of course it is. The rule has something distinctly akin to making a Polish chicken soup*. First thing you do is steal and perhaps adapt a bit so you don't get sued for plagiarism, quite a few hundred recipes and then put your book together. Originality - for sure not but what the heck, the printing press really doesn't think much about that.
*With apologies for what might be a bit racist but it is, after all, the standard joke.