foxtail22 said:3. The number of RS485 bus loads. The base ELK supports 4, you will need a bus expander to add more than 4 devices and each keypad counts as do the zone expander cards.
The M1 supports more than 4 data bus loads. Although you can connect only two daisy-chained data bus runs to the M1, each daisy chain can have many devices on it (keypads, zone expanders, etc). If everything is wired as a home run with CATx cable, you can create the daisy chain with M1DBHs, or wire it by hand without a DBH.
Keypads and zone expanders are devices are of different types. You can have up to 16 keypads (which are type 1) and 13 M1XINs (type 2, limited by total zone count). Output expanders are type 3. So a keypad set to address 2 is different from an M1XIN set to address 2, since they are of different types.