Well the pattern should give a definition for psso. It's generally not used by itself, but as part of a sequence to decrease stitches, such as slip 1, k1, psso.
That's a decrease, and so is k2tog, so at the end of row 15 you will have decreased 2 stitches and have only 19 remaining. The st number on row 7 will be decreased by 2 sts also. There's no left over sts - you'll be left with less sts means there's that many remaining.
On row 7, you start with 29 sts and end up with 27 when you're done. On row 15, you begin with 21 sts and have 19 at the end of the row.