Probably not very many. According to XKCD What If? [1] a modern keyboard takes around 2 millijoules to press a key. Typing a full novel would take a few kilojoules. Even if an old mainframe keyboard took 10x more power to press the keys you would save less than a AA battery worth of energy over writing a full novel.
Using some conversions from an internet site, one AA battery is 1.3e4 Joules and a human requires 8.4e6 Joules per day, so about 133 seconds of energy saved per 6 months of novel, or two lost seconds of calorie burning exercise every three days.
[1] https://what-if.xkcd.com/102/