T9 is a predictive typing system, but for fewer keys. How to calculate?

Question

Keyboard layout

In a recent thread about the 5-key layout, an idea was suggested in the comments that you can make a keyboard with the principle like T9 , but with 5 buttons.
Actually, such a keyboard should be called T5. I have a question, what is the minimum number of keys required for this input method for a particular language? Let's start with Russian and English. Letters do not have to be in alphabetical order. Any chance to calculate for example 3-button or even 2-button layout?

Answer 1

one / two-button already exists - the telegraph had one (Morse code).
but it’s hard to understand why this is, if a person can work 10 fingers at once)