It's not about numbers per se. How do you type `~!@#$%^&*()'-_=+? What about Esc, Tab, Backspace, Enter, Ctrl, Fx etc.?
Another thing to keep in mind… Someone has recently linked a study that compared hardware and touchscreen keyboards—with obvious results: touchscreens allow effortless keystrokes, but at the cost of significantly increasing wrist strain, because fingers can't be "rested" on keys.
In this context, keyboard-less input methods are worth consideration. Something like MessagEase, Dasher, etc.