What matters is not just the number of fingers, but how easily they can move their fingers and in what directions. And which fingers are stronger and which are weaker.
For example, if they can easily flex the left most finger to the left and the right most finger to the right, then you could have a keyboard with at least 12 keys across: They normally position their left hand over columns 2 through 5 and their right hand over columns 8 through 11. Then they reach column 1 by moving their first finger slightly to the left, etc.
If moving fingers left and right is hard but they can readily move forward and back, then make it just 8 columns wide but have as many rows as you need.
If they have very limited movement of their fingers in either direction, maybe the keyboard has to be divided into 2 "banks". Have an upper bank of, say, 8 columns and 3 rows, and similarly a lower bank. To switch between banks they have to move their hands up or down rather than just moving fingers.
Other option: Have shift keys or other multi-key combinations. Lets suppose they can only easily handle 8 columns by 3 rows = 24 keys. Assuming they need more than 24 characters, make one or more of those keys shift keys, and you need to use shifts to get to the less common letters. Life if they are trying to type English, maybe make "Z" be "shift-S" and "W" be "shift-V". I mean a different shift from the capital letter shift, just something to get you to an alternate set of keys.
The other obvious possibility is that they don't "touch type". The keyboard is too big for them to put their hands in one position and reach any key, so they just hunt and peck all the time, like human beings who never learned to type.