You need to pick up 20 stitches total across the 17--so you add in 3 extra stitches. Usually you space these out evenly across the total length (in your case over the 17 stitches).
Here's a video about picking up stitches: https://www.youtube.com/watch?v=xmy0...eature=related
At about the 2:16 mark, the man talks about picking up two stitches per "row" (sic)--actually it's not per row it's more like per "column."
See if this helps answer your question.