Canon MX520 series Fax Manual page 74

Note
The machine can store up to max. 50 pages* (max. 20 documents) of faxes in the machine's memory.
* When using ITU-T No.1 chart (Standard mode)
If the machine's memory becomes full during memory reception, only pages which already have been
received are stored in the memory. Print the received faxes stored in the machine's memory or save
them on the USB flash drive, delete them from the machine's memory, and have the sender resend the
faxes.
Document Stored in Machine's Memory