How many ways can a batter be credited with an RBI?

Answer 1

A batter can get a hit.
A batter can walk.
A batter can get hit by a pitch.
A batter can run out a drop third strike and make it safely.
A batter can reach first base on an error.
A batter can reach first base because of a fielder's choice to make a play elsewhere.

Answer 2

A batter can be credited with an RBI for every run that scores because of his hit, sacrifice bunt, sacrifice fly, infield out, fielder's choice, walk, and when he is awarded first base because of a hit by pitch, interference, or obstruction.

If there are fewer than two outs, a batter can also be credited with an RBI after having reached on an error, so long as the run scored on the play was not a result of the error.