How many Diary of a Wimpy Kid books are there?
About 20 books in the main series. The count changes when a new one comes out, so check the publisher's site for today's number.
The main Diary of a Wimpy Kid series has about 20 books, written and drawn by Jeff Kinney. Sources disagree by one or two, because the number changes when a new book comes out and some lists count a special book or a new spin-off series differently.
There is also a separate spin-off series, Diary of an Awesome Friendly Kid, which is not part of the main count.
How to get the exact number
The author's website and the publisher's website always list the books in order, with the newest one at the top. A grown-up can check it for you, or your library can show you the whole series on its shelves.