Hello and sorry you're having problems. There is no way to calculate how many books the E-reader will hold- since all books are of different length. However the reader should be holding upwards of 1000 books.
If the Reader dropped a book, when there were only 220 average length books, then it's possible the reader simply needs to be reset:
IMPORTANT: A soft reset can be performed anytime and will resolve most issues.
There is also local memory in the device in the form of an SD card- so even if the reader is full- you can remove the card, and insert a new one then begin storing onto that card. The reader can accept up to a 32G SD card which can be purchased at Best Buy for about $20:
The following link offers some notes regarding the SD card on this model Reader as well:
If you need further assistance let me know. Please do not forget to click a POSITIVE RATING before you go- as it's how I am credited for my work. You can follow up at any time, at no additional cost.
Hello. My name is***** can provide you with some additional assistance.
Sony claims that this model will hold up to 1200 books, but the actual limitation is based on the 2 MB of memory. So the number of books will vary based on how large or small the books are in the size of the file. There is no finite limit on the number – it's actually capable of storing in excess of 65,000 items, so the number of documents is not the limiting factor – the memory is. So it is possible for it to be full with 220 books. It just depends on the size of the files.
This device does have the ability though, to take additional memory in the form of a microSD card. You can get microSD cards that are fairly large in capacity, so if the size is limiting you a 32 GB microSD card for example, would allow you to store 16 times the number of books that the basic memory allows you to store.
Please let me know if you still questions.