It will depend on how much information each of the 20 slots will need to store. You could just divide the 64KB by 20 and each user gets that chunk. The problem is that you'd have to keep track of writing across pages. If you write a single byte at a time (rather than 32 bytes at a time), that wouldn't be an issue but it will take longer.
You can divide it up so that each chunk starts on a page boundary but you may end having some unused apace in the EEPROM and not have enough room in the user chunk to store the data.
Share with us how much space each of the 20 users needs and we can come up with a better solution.