It really depends on the size of the books you're trying to store. Generally speaking, an 8GB Kindle can store thousands of books, but this could range from a few hundred to tens of thousands, depending on the size of the books and other files.
For example, if you store mostly standard Kindle books, you can typically fit around 3,000 books in 8GB of storage. If you are dealing with larger books, such as textbooks, cookbooks, and graphic novels, you could fit closer to 1,000.
The best way to get an accurate estimate of how many books you can store in 8GB of Kindle storage is to use Amazon's free Kindle Storage Calculator. It will allow you to quickly determine how many books of a certain size and format can be stored in a given amount of space.