How many frets should the fingers cover in one position?
Because the frets on a mandolin are closely spaced it is easier to hold the hand fairly "square" to the fingerboard and depending on the scale being played, essentially use first finger for frets 1+2, second finger for 3+4 third finger for 5+6 and "pinkie" for fret 7 if needed (sometimes it makes sense not to move to the next string up).
On a tenor banjo, which contains the same note intervals but on a longer fret scale you have to move your hand to cover the scale, so the fingering tends to be tackled in a slightly different way.
Quack!
GtD.