This is a difficult question to answer as everybody’s reading speed and habits may differ. What’s a good number to read can be different for each person depending on their preferences.
If you want to get the most out of your reading habit and make it a regular habit, then the most important things is to find the right balance and a target number that you can realistically achieve.
I usually set myself the goal of reading two books a month, or 24 books in a year. This gives me enough time to appreciate each and every book, as this way I am able to read, understand and really absorb what I am reading. Also, this keeps away the pressure to read more when I feel like I am running out of time.
On the other hand, if you are aiming for more, like reaching for a certain educational goal or a longer challenge, I'd suggest going for one book a week, which should add up to 52 books a year. That way, you can have a bit more room for the books that require a longer, more dedicated read.
Ultimately, the best number of books to read a year would be the one that keeps your reading habit enjoyable and manageable.