The distance from each plant (row) depends on size of your crop, what it is, and how steady a' hand you have on the steering wheel.
Long straight rows of 3" tall corn, you can set the "sweeps" or "shovels" as close as a couple inches. It takes tremendous concentration to NOT plow out your corn crop though. Most people are better off staying 4" to 6" away.
Old cultivators are a dime a dozen (at auctions or implement dealers) It's a piece of cake to cut down a 4,6, 8, ect... row unit to make a 2 row model. Then you have extra parts for the NEXT project too.
Cultivating (corn, soybeans, ect) is a tedious job. You need to keep enough speed to make the shovels work correctly. That means plowing out a few rows of the crop from time to time. (BTDT) It takes practice. (And my father made sure I got LOTS of practice growing up /forums/images/graemlins/tongue.gif)