The maximum combined number of buttons on button modules per system is 1024.

BM12: +24 (Max 3 per phone, 42 per system)
Add-on for 9508 phones. Provides 12 physical buttons assignable in 2 pages for 24 programmable buttons in total. When used with a 9508, power must be supplied to the phone using an inline power module.

KLM: +48 (Max 2 per phone, 21 per system)
Used with M7324 phones. Up to 2 buttons modules are supported per phone. Each button module provides 48 programmable buttons.

T7316E KEM: +24 (Max 9 per phone, 42 per system.)
Used with T7316E phones. Each button module provides 24 programmable buttons. Up to 4 buttons modules are supported per phone without additional power supply.