There are three pages of programmable keys. The T41P and T42G have 12 keys, while the T46G and has 22 keys.
By default, only Page 1 will be visible. If you configure any keys on Pages 2 and 3 using the Portal, the final key will become a Next Page key.
The SPA504G has 3 customisable keys, the SPA508G has 7 and the SPA525G has 4.
Gigaset cordless does not have any programmable keys.