2 = n
There are two down strokes in a typewritten n.

3 = m
There are three down strokes in a typewritten m.

4 = r
The word "four" ends in r.

5 = L
Your five fingers, with thumb out, form an L.

6 = j, ch, sh, soft g
There are six letters in shucks, chucks, and j and g have a round base & a similar pronunciation.

7 = k, hard c, hard g
moving two seven's around, you get a k, hard c and hard g have similar pronunciation/sounds.

8 = f, ph, v
handwritten f looks like an 8 and f, ph and v have similar pronunciation/sounds.

9 = p or b
hold 9 up to a mirror and you see a p and b and p have similar pronunciation/sounds.

0 = soft c, s, z
the final digit is 0 and the final letter is z and c, s and z have similar pronunciation/sounds.